Category Archives: Electrical Topics

Lithium Batteries On Boats – Part 2

8/13/2021: Initial post

Introduction:

Many boaters have been lead to think that lithium chemistry batteries are just an efficient energy storage technology that can easily replace existing lead acid batteries in a boat.  For 100+ years, boat DC “electrical systems” have evolved to be inextricably compatible with the characteristics and behavior of lead-acid batteries.  Lithium chemistry batteries are protected and controlled by internal, “semi-intelligent” BMS circuitry which is not inherently compatible with lead-acid “electrical system” designs.  Incompatibilities expose some reliability gaps with traditional “electrical system” designs which become concerns when lithium chemistry batteries simply “replace” lead-acid batteries.  The guiding principle in battery replacement must be that a reliable electrical system is essential to crew and vessel safety on any boat.  “Electrical system” designs that are fully compatible with lithium chemistry battery characteristics are yet to be fully developed, tested, certified and adopted in order to assure “electrical systemreliability with LFP battery technology.  Issues and concerns are not about the batteries; it’s about the “electrical systems” into which the batteries are fit!

This article, like the previous article, grew quickly and became quite lengthy.  These twin articles are an “orientation” to using lithium chemistry technology on boats that just skim technical design detail.  The scope of this subject fills engineering textbooks.  Just as all of the organs of the body have their own complex internal functions, each must work harmoniously with all of the others for the health and wellbeing of the human host.  Doctors work with hearts, lungs, bones, nerves and muscles as component parts that make up a complete, living body.  Engineers, service technicians and skilled DIYers work with batteries, wires, switches, motors, appliances and instruments; all of the parts that make up a complete boat “electrical system.”   For the system to be healthy and RELIABLE, all of the parts must work harmoniously together!

Profile of Today’s Recreational Vessels:

There are three principal factory configurations for the DC electrics on cruising boats manufactured in the last 20 -30 years.  Most recreational boats in the 30’ – 65’ range will be in one of these cases:

    1. boats built with a single, central battery bank that “does everything.”
    2. boats built with two battery banks, one that starts the engine (“Start”) and does nothing else, and a second one that does everything else (“House”), or
    3. boats as in two, above, but fit with a third battery bank dedicated to onboard inverter(s) which in turn power some or all onboard AC loads.

Variations of the above are certainly found;

    1. separate “start” banks for multiple engines
    2. “house bank” split into two parts, Port and STBD; fore and aft,
    3. separate “not start bank,” “not house bank” for thrusters or windlass or winch.

Most commonly, battery banks on mid-sized boats built for lead-acid batteries are 12V or 24V configurations.  In some cases, both 12V and 24v banks will be found on the same boat.  Infrequently, 32V system may be seen.  Below, following, 12V systems will be the baseline of discussion.

Case 1: Central Battery Bank

The DC portion of the “electrical system” aboard our Sanctuary has a single “central bank” as shown in Figure 1.  This approach maximize the utilization, efficiency and service life of our single battery bank composed of six lead-acid flooded wet cell, 6V golf cart batteries (2S3P) and spreads normal aging and lifetime loss-of-capacity progression evenly across all of our individual batteries.

This single 12V bank runs our house loads, inverter loads and starts our propulsion engine.  This configuration is efficient, uncomplicated and economical to own and operate.  It requires a single AC charger to charge batteries from shore power or our onboard 120V genset.  Our AC charger is a Magnum MS2012 inverter/charger rated at 100A DC charging current.

Sanctuary is a single-engine trawler with one engine-driven alternator.  When underway, the alternator provides 12V B+ power to our DC system and charges our batteries.  The alternator is a 12V, 110A Balmar (Prestolite) small-frame machine fit with an external Balmar 3-stage (bulk, absorb and float) voltage regulator.  Sanctuary is also fit with a Magnum Model BMK Coulomb-counting battery status monitor to enable tracking of battery voltage, load current and battery State-of-Charge (SOC).

Since there is only one central battery bank aboard, there is no “1-2-both-off” battery switch, no duplication of power distribution wiring, and no concern for resistive losses in the DC cabling that makes up the battery bank power distribution circuits.  Monitoring and maintenance are easy.

Over the years, I’ve recommend the above approach for its simplicity, utility and reliability.

The principle “perceived weakness” to Sanctuary’s consolidated, central battery bank is “lack of redundancy.”  The concern centers on the idea that if our single bank were to fail or become sufficiently over-discharged, the boat could be left unable to start its propulsion engine.  Since we have an onboard genset, that would be only a minor inconvenience, but in an inadequately monitored and managed system without a genset, over-discharge could happen.  To a boat manufacturer selling into a generic consumer market, that risk is a valid consideration for the design of a boat “electrical system,” so a second “start” bank is sometimes added by boat builders.

Case 2: Separate “Start” and “House” Banks

In Figure 2, the DC portion of the  “electrical system” shows one battery bank that is dedicated to starting the propulsion engine and a second battery bank that is dedicated to all other DC loads.  The “start” bank only powers the engine’s starter motor and the engine’s control circuits, sensors and gauges (fuel solenoid, coolant temperature, oil pressure,  exhaust high temperature alarm and gauge illumination), but nothing else on the boat.  In the lead-acid world, “start service” batteries are specifically intended to give up the energy they store in very large, short-duration bursts.  “Start service” batteries do not store much total energy.

“House” loads comprise “everything else DC” on the boat (bilge, water and waste pumps, deck washdown pump, navigation instruments, windlass, thruster, inverter/charger, refrigeration, computer and entertainment equipment and lighting).  In the lead-acid world, “house” batteries are “deep cycle” batteries intended to hold a great deal of energy, but to give it up over long periods of time.

Redundancy paths are depicted in Figure 2 by the dashed blue lines, showing there are two possible DC sources that can alternatively (redundantly) be selected to power the engine’s starter motor.  If either path were to be unavailable/fail, the other would be available as backup.  Distribution wiring and switching detail is not shown in this “conceptual diagram.”  In return for the “operational redundancy” that this dual bank system is seen to provide, the addition of a separate “start” bank adds cost in the form of batteries, additional switching and cabling, and charging equipment.  Additional batteries and equipment also adds ongoing monitoring complexity and maintenance for the boat owner.

Case 3: Independent “Start,” “House” and “Inverter” Banks

The DC portion of the “electrical system” in Figure 3 shows separate, independent battery banks for:

  1. “starting” the propulsion engine,
  2. “house” DC loads, and
  3. AC “inverter” or “inverter/charger.”

This approach provides additional power redundancy (blue dashed lines), but brings with it additional equipment cost, wiring and switching complexity, owner monitoring and maintenance overhead and the greatest service life compromises for evenly balanced battery utilization.

The benefit of a separate battery bank to power an inverter(s) is because AC loads (microwaves, cooktops, toaster ovens, washer/dryers, ice makers, freezers, watermakers, AC davit winches, etc) create very large DC amp draw from the batteries that feed the inverter.  In 12V systems (ignoring inverter efficiency), the ratio of 120V AC to 12V DC current is 10:1.  So 10 Amps AC drawn from the inverter to power a toaster will exceed 100 Amps DC drawn from “inverter” bank batteries.

High-demand transients cause lead-acid battery terminal voltage to “sag.”  High demand loads also invoke “Peukert’s Law,” which describes the phenomena where lead-acid batteries that are discharged faster than their design rate (20-hours in North America) will not return as much of the energy as they could return if discharged at or more slowly than their design discharge rate.

A separate battery bank for the AC inverter and its AC loads isolates DC “house loads” from DC voltage sag caused by AC equipment cycling “on” and “off” at random intervals.

Electrical System Compatibility Considerations

Lithium chemistry batteries rely on a BMS to keep their electrochemistry operating within safe parameters.  The BMS is an autonomous, semi-intelligent component inside the battery’s case that is able to independently, suddenly and without warning “decide” to disconnect attached DC loads.  The BMS will electrically disconnect the batteries from the circuits they are intended to power in the event of internal electrochemical or thermal threats with continued battery operation.  As a result, “electrical system” designers face previously nonexistent compatibility concerns with lithium chemistry power.

With lithium chemistry batteries, it must be decided early in the process of designing the “electrical system” which DC loads can tolerate a sudden power loss and which loads cannot tolerate such a power loss without potential safety impact to crew and/or vessel.  This analysis affects everything about the layout and composition of the electrical equipment on the finished boat, as well as the electrical cabling needed in the boat-build process.

In the past, boat designers have not had to perform this analysis.  In lead-acid “electrical systems,” it is entirely acceptable to assume that all DC loads will have DC power all of the time.  In lithium chemistry systems with mission-critical loads that cannot lose DC power without risk to the safety of crew and vessel, alternative means of assuring continuous power availability to mission-critical circuits must be provided through the intentional design and implementation of the “electrical system.”

When converting from lead-acid to lithium-chemistry batteries aboard a boat, this same safety analysis  must be done early in the conversion project plan.  Since the boat already exists, and is fit with an existing lead-acid based DC system, a retrospective engineering safety evaluation is needed, starting with an inventory of all of the electrical equipment on the host.  Once the inventory is complete, priorities based on importance to crew and vessel safety can be assigned.

The evaluation begins by listing ALL of the equipment aboard the boat.  Then, considered judgements about the safety role and resulting operational priority for each specific listed item are made.  This evaluation includes consideration of how the boat is actually used by its owner: ex., single-hander vs crew aboard, live-aboard full-time cruiser vs periodic weekend and vacation vs marina maven, frequency of low viz ops, night ops, international offshore ops (ex: Miami to the Bahamas), near-shore offshore ops (avoid crowded conditions on the A-ICW), seasonal layup periods, regional weather patterns, seasonal river flooding levels, or other factors that apply to the safety of crew and vessel.

By way of example only, Sanctuary’s equipment list and prioritization might look like this:

Circuit  Priority   Notes
Salon compass illumination 3 Multiple sources of console illumination available
Anchor light 2 Multiple portable backups
Navigation lights 1 Dusk-to-dawn, low viz
Deck Lights (downlight) 3 Portable lighting available
LPG Control Valve 3 Not used when underway
Engine Room lights 1 Must work
Cabin Lights 1 Must work
Aft Cabin Lights 2 Not essential
Windshield Wipers 2 Very rarely used
Horn 1 Must work
Bilge Pump 1 Automatic; must work
Bilge Pump 3 manual
“High Bilge” Alarm 1 Indicates potential emergency
Sump Pump 1 Automatic – doubles as bilge pump
Sump Pump 3 manual
Potable Water Pressure Pump 1 Manual pump not installed
Anchor/Deck Washdown Pump 1 Doubles as a fire-fighting option
Refrigerator/Freezer 1 12V/120V model
12VDC utility outlets (flybridge) 3 iPhone, Sirius/XM radio
12VDC utility outlets (salon) 3 TV Antenna
Head(s) 3 Electric macerator with manual backup
Salon AM/FM Radio 3
Windlass 3 Manual ops available, feasible
DC-to-AC Inverter 1 3 Nav table items: Printer, TV, DVR, DVD
DC-to-AC Inverter 2 1 All boat AC Outlets (computer, router, USB Hub, iPad, iPhone, AIS Transceiver, space lighting, crockpot, coffee maker, microwave)
GPS 1 (Garmin) 1 Flybridge (DSC 1)
GPS 2 (Raymarine) 1 Salon (DSC 2)
VHF Transceiver 1 (DSC 1) 1 Flybridge (Foghorn)
VHF Transceiver 2 (DSC 2) 1 Salon (redundancy)
Depth Sounder 1 Flybridge
Chart Plotter 1 (Autopilot) 1 Flybridge
Chart Plotter 2 (RADAR) 1 Flybridge
Chart Plotter 3 3 Salon; redundant iPad, iPhone apps
AIS Receiver 3 non-essential
NMEA Data Multiplexor 1 Supports iPad, iPhone apps
Compass 3 Analog; multiple redundancies
Propulsion Engine Systems 1 Key switch, alarm sounders, gauges, temp & oil pressure sensors, console instrument illumination
Thruster Control System 3 Not used underway; manual maneuvering feasible
N2K Instrument Backbone 1 Autopilot ops, incl display heads
Autopilot components: ECU/CCU and Hydraulic Pump 1 Autopilot ops
Amateur Radio Transceiver 3
SSB Antenna Tuner 3
Boat Monitor 1 Bilge pump cycles; battery voltage; fridge/freezer/salon/engine room temps

About LiFePO4 Batteries:

Buyer beware!  Caveat Emptor!

Virtually all LFP battery components originate in China.  It is well known that product quality from the various cell manufacturers varies widely.  The primary market for lithium chemistry batteries world-wide is not recreational boating in developed countries.  Recreational boating is very much a secondary market.  The cells sold into secondary markets are often “seconds” and “blems” that do not meet the quality, consistency and capacity standards requirements of the primary markets.

Any “Made in the USA” claim by ANY lithium battery manufacturer/seller is unlikely to be “the whole truth and nothing but the truth.”  LFP cells and BMS circuit boards are made in SE Asia and shipped to the US.  For some brands, it’s acceptable to see “Assembled in the USA.”  Warrantees can be misleading; a 10-year warranty only covers “manufacturing defects,” NOT “cycle life” promises.

Manufacturer’s battery specifications are very, very important.  “The devil is in the details!”

There are no formal safety testing standards specifically targeted to LFP batteries.  One UL/cUL Standard in the North America that includes LFP batteries within its scope is UL1973.  Equipment fails.  Mistakes and accidents happen.  LFP batteries are no exception.  UL does aggressive testing, including vibration, severe over-charge and over-discharge, and short-circuit testing.  Certification testing of this kind is very expensive for manufacturers, so very few LFP batteries carry UL’s listing mark.  Retail pricing for those that do reflects testing costs.  Batteries listed to UL1973 are unlikely to suffer catastrophic consequences caused by human error or component failure.

LFP Battery Characteristics:

A mandatory task in evaluating lithium chemistry batteries is to carefully review and consider the manufacturer’s performance characteristics of the batteries, themselves.  This task is quite challenging, even for trained and experienced electrical practitioners, because laboratory technical reports often differ in significant ways from each other and from the claims made by battery manufacturers in brand-specific promotional literature.  Further, the claims made by any one individual battery manufacturer are often at variance from the claims of industry competitors.  It’s often necessary to review multiple information sources, and “average it out,” based on “conservative best guesstimate.”

The following table is a compilation of data assembled from many sources, but not an absolute list of all LFP technical characteristics.  Individual manufacturers often claim different values for comparable operational factors.  For example, the safe operating amp and temperature ranges for charging LFP batteries is significantly more narrow than the safe operating amp and temperature ranges for discharging them.  Comparisons are difficult, sometimes imprecise, but there is enough in this table to help identify and consider some important “electrical system” design questions and choices.

There is a great deal of technical detail behind all of these comparison factors, and it’s neither practical nor reasonable to expect lay buyers to understand them all.  But they’re there, and they do affect risk, reliability, lifetime cost-of-ownership and value.  To avoid cost, disappointment and risk, a pre-conversion understanding of the major technical details is important.

One reason commonly cited for the apparent desirability of LFP batteries is the comparison of amp hour capacity of lead-acid batteries to LFP batteries by usable energy percentage, weight and volume.  The comparison implies that buyers can allocate less weight and total space to the more energy dense LFP batteries.  True?  Maybe.  In this discussion, buyers must diligently avoid “wishful hearing.”  The above conclusion works for “amp hour” capacity, but there’s more to batteries than amp hours.  Consider: “amps.”

Amp Hours and Amps ARE NOT the same thing.  “Amp hours” is a measure of energy storage capacity (ex: a fuel tank able to hold some fixed number of quarts or gallons of fuel; it’s “capacity”).  “Amps” is the measurement of the rate at which energy is consumed (ex: 6.0 gallons-per-hour of fuel consumed at “hull speed” vs 28 GPH consumed at “planing speed”).  Terminal voltage of lead-acid batteries “sags” when the battery is giving up high amps.  Terminal voltage of LFP batteries doesn’t sag, but thermal heating occurs as high amp draws consume energy, with the possibility the BMS will trip and disconnect the battery from its load.

PbSO4                                                   LiFePO4            
1 Technology Wet Cells AGM & Gel Carbon Foam enhanced AGM Lithium Ferro-Phosphate; LFP
2 Weight Heavy 1/3 weight of PbSO4 per aHr of C-Rating
3 Orientation dependent Yes; liquid electrolyte No – sealed No
4 Maintenance Needs “watering” and equalization None – captive electrolyte None (with BMS)
5 Service Life Baseline of 1.0X assumed; ±300 charge/ discharge cycles in lab conditions; ~150 in real life 4X – 10X PbSO4; shorter cycle life if deeper avg discharge
6 BMS HVCO n/a 3.65V/cell
7 BMS LVCO 2.50V/cell
8 BMS HTCO >135℉ charging1;

>160 ℉ discharging

9 BMS LTCO <25℉ charging; <-4℉ dischg
10 Energy Density Lowest per unit weight & volume;

35 wH/kG

Slightly better than wet cells;

40 wH/kG

Slightly better than AGM 3X >PBSO4 by weight & volume;

115 WH/KG

11 Recommended Discharge – SOC 35% – 40% (Pessimistic?) 50% (Industry rule-of-thumb) 65% (optimistic?) 80% – 90%
12 Max discharge (volts/cell) 1.75V Consult Manufacturer 2.5V
13 Max discharge amps n/a 0.5C – 1.0C
14 Cost per aHr Lowest per aHr returned 2X – 3X wet cells 1.5X – 2.0X AGM 3X PbSO4; TCO is < PbSO4 over rated cycle lifetime
15 Voltage Curve Sags with ↓ SOC Sags slightly < wet cells Sags < AGMs Nearly flat from 100% ↓ 15% SOC
16 Charge Time Baseline 1.3X faster 1.6X faster 4X faster
17 Equalization necessary Yes Some can be No No; “voltage balancing” done by BMS
18 Subject to sulfation Yes No
19 Can be stored partially charged No Yes; prefers 60% SOC for storage
20 Needs “Floating” Yes Consult Manufacturer No
21 Self Discharge Moderate Low Very Low
22 Sensitive to over-charging Least Slight – AGM; Yes – Gel Consult manufacturer Yes; BMS required
23 Sensitive to over-discharging Yes Least Yes; BMS required
24 Capacity  (C)  vs Discharge Rate 100% at 20 hr rate
80% at 4 hr rate
60% at 1 hr rate
Consult manufacturer No Peukert effect
25 Charge Temperature Correction (“Slope”)2        Deka: -3mV/°C/cell
Crown: -3mV/°C/cell
Concorde:-4mV/°C/cell
Surette: -4mV/°C/cell
Exide: -5.5mV/°C/cell
Ref: IEEE 450-2010
Consult manufacturer None
26 Conversion Costs (PbSO4 → LiFePO4) n/a Highly variable; depends on choice of platform, incl modifications & reconfiguration of existing host electrical system
27 Releases Hydrogen when charging4) Yes Sealed – VRLA No
28 Releases Chlorine gas if submerged Yes Sealed

Table Note 1: May be too low to accommodate placing the battery bank in the engine room.

Table Note 2: The industry-wide baseline reference temp for lead-acid batteries for temperature correction is 25°C (77°F); the normal, permissible operating temperature  range is considered to be 5°F – 130°F.

Load Profile – Start Bank:

All starter motors draw very high DC currents.  Sanctuary’s 4-cylinder, 150hP diesel has a 12V Delco starter motor with an inrush current of 380A and a steady-state running current of 250A when cranking over the engine.  Bigger engines may have greater starter motor power demands.   Newer reduction gear starter motors may have lesser amp draw demand.  Cold engines crank longer than warm engines.  Engines with glow plugs may need additional DC power to “light the glow plugs.”

To simply start a healthy engine, starter motors don’t have to crank very long in normal operation, but periodically – as when re-priming a diesel fuel system after major maintenance – they do need to crank for prolonged periods.  Lead-acid start service batteries can meet all of these needs because they can release high amp draws very quickly without concern for overheating.

LFP batteries don’t come in “start service” and “house service” models.  LFP batteries are rated in amp hour capacity (often, watt-hour/kilogram, or Wh/kG), also known as “C-Rating.”  LFP batteries may or may not be a suitable choice for high-current starter motors.  Generally, the “industry guideline” for LFP batteries is to limit amp draw to 0.5C.  Many marine manufacturers limit their second quality or other-than-new LFP cells to 0.3C – 0.4C.  To prevent BMS disconnects or cell damage, and maximize cycle life, a lithium battery with a 100 amp hour (aHr) C-rating should be limited to loads of no more than 40A – 50A.   An LFP battery rated at 300 aHr should be limited to loads of no more than 150A.

The math of “amp-hours” confirms that it doesn’t take a lot of total energy to start an engine.  Figure 4 shows an actual measurement of Sanctuary’s starter motor inrush current, in Amps.  Our engine takes less than one aHr of energy capacity to start it (one Amp Hour being one amp drawn for one hour).  This math very conservatively assumes 380A inrush is sustained throughout a 5-second-long duration starter motor cranking time; most healthy engines start in 2 seconds, ±:

aHr = (A x Hr)
= (380A x 5 seconds)
= (380A x 5/3600 Hr)
= 0.53 aHr.

It’s clear Sanctuary doesn’t use much total stored energy, but does need 380A to spin the motor against the mechanical load of cranking the engine.

In consideration of the above, can the lithium chemistry battery tolerate prolonged engine cranking load?  Answer: maybe.  It depends on the ratio of LFP battery C-Rating to starter motor amp demand; that is, “how much larger is the starter motor amp draw than the discharge rating of the LFP battery/battery bank?”

Sanctuary needs less than 300 aHr of usable energy for a winter overnight (14 hours of darkness) at anchor, but she needs 380 inrush amps and 250 running amps to crank the engine.  With 400aHr of LFP battery capacity, there would be plenty of energy for us to overnight on the hook, but when cranking, would be well in excess of the 0.3C – 0.5C current draw rating of most LFP batteries.  All LFP batteries will have a short-duration “overload margin;” perhaps 1C overload for 30 seconds, 2C overload for 5 seconds or 10C overload for 500 mS, so it’s tempting to conclude that a 400 aHr LFP battery would be acceptable for Sanctuary‘s starting needs.  But, the next time the fuel system needs to be re-primed after changing the secondary fuel filter or servicing the injectors, we might be faced with a nasty surprise.  That service activity would require prolonged periods of engine cranking load, and surfaces as an obvious “electrical system” reliability concern.

Pro Tip: the larger the discharge rating of the battery in proportion to the amp demand of the starter motor, the better its suitability for use as a “start” battery.  In considering this design question, keep in mind that there will be times when cranking will be required for continuous minutes.  That happens only infrequently, so that’s the time when inadequate design is most likely to surface as an issue.

Balancing the Cell Packs

One further consideration is how the needed amp hour capacity will be obtained:

    1. a custom-designed high C-rating battery with a BMS matched to the needs of that assembly (i.e., 600 aHr, 3P4S configuration of 3.2V, 200 aHr cells), or
    2. a parallel collection of 3 x 200 aHr “drop-in” LFP independent units.

Option 1 may be adequate to handle the starter motor, but option 2 may not.  “Drop-in” batteries are more susceptible to being individually overloaded when arranged in a high-load parallel circuit where small differences in cable length and connection integrity are typical, introduce unequal resistances, and result in unbalanced currents in the individual battery contribution to the total load.  The net is, absent significant “electrical system” design, starter motors are not well suited to being powered by lithium chemistry batteries of modest or conservative C-rating ratios.

As with inverters, a design alternative is to upgrade “start” bank operating voltage.   Doubling the voltage of the bank from 12V to 24V cuts the amp draw requirements in half to get the same kW of energy.  That choice isn’t always practical.  For example, there would have to be a 24V starter motor that would fit on the existing engine.  Engine controls like fuel solenoid, oil and temperature sensors, gauges and instrument illumination would have to be considered; these devices could be changed to 24V or remain wired for 12V.  Conversion cost details then arise about how to charge the 24V battery.

Pro Tip: Electrically, windlasses, winches and thrusters all contain DC motors with the load profile of starter motors.  They draw very high currents for variable, but generally short-duration, bursts.  A thruster running for 20 – 30 seconds may be harder on LFP battery internal heating than a much larger engine starter motor running for only 2-3 seconds.  But, 1C for 30 seconds might not cover the runtime needs of a foul windlass, or a thruster in abnormal conditions of wind or current, or a winch with a dinghy full of gear or rainwater.  Boat owners must decide if these are conditions where loss of battery by BMS Disconnect would be acceptable.

Load Profile – House Bank:

“House” is a “catch-all” term that tends to mean “everything else not starter motor.”  That has its origins in the differences between lead-acid “start service” batteries and lead-acid “deep cycle” batteries.  “House loads” are DC loads of more modest magnitude and longer duration, supported by “deep cycle” batteries.

Toward the goal of designing a lithium chemistry system (a reliable electrical system that is safe for crew and vessel), equipment that makes up the “house loads” has previously been inventoried and individual loads have been classified as “high,” “medium” or “low” priority for needing to be continuously powered.  High priority equipment needs battery systems that are 100% reliable.

With lead-acid batteries as the underlying design assumption, all of the “house stuff” was traditionally powered from one, common B+ buss.  Conversion to LFP batteries may require installation of net new B+ buss(es) to power a high priority load group(s), and/or may require re-configuration (subdivision) and re-wiring of some existing circuits.  In evaluating existing B+ power wiring, owners need to consider whether some very high priority equipment shares a circuit breaker that ALSO POWERS lesser priority equipment.  When Sanctuary was built, the electrical cables that brought DC from her battery bank to her power panel and from her power panel to her branch circuit load locations never considered anything other than lead-acid batteries.  As a result, our high priority flybridge and salon nav instrument B+ feeds also include lesser priority device attachments.

The obvious “mission-critical” equipment on an existing “house” bank is the Navigation Electronics (depth sounder, chart plotter, VHF Radio, autopilot, RADAR, AIS and “fly-by-wire” steering and throttle controls).  Secondary loads like navigation lights, horns, windshield wipers and refrigeration are important, but perhaps not “mission critical.”  Tertiary loads like LPG gas valves, inverter/chargers, water makers, fresh water pumps, waste pumps, washdown pumps, space lighting, deck lighting and accent lighting are lifestyle and convenience items, but certainly not “mission critical.”

Load Profile – Inverter Bank

Typically, AC equipment on a boat includes things like induction cook tops, microwaves, coffee makers, toasters, instapots, crockpots, blenders, ice makers, fans, TVs and other entertainment equipment and computer equipment.  Mostly, this equipment is not “mission critical” to safety; lifestyle, convenience and comfort, certainly; but, not safety.

What about AC medical equipment like BPAP/CPAP, WoundVac or infusion pump?  Medical equipment isn’t mission critical to the vessel, but it certainly can be to affected crew member(s).  If there are “mission critical” AC appliances aboard, they must have 100% reliable AC power.

Pro Tip: Stand-alone inverter battery banks can be upgraded to 24V, reducing battery cable size requirements and moderating battery voltage sag related to transient AC loads.

Pro Tip: AC power can optionally be accomplished with a collection of smaller inverters distributed around the boat rather than one, large central inverter.

Pro Tip: AC powe large inverter battery bank may be able to support both a large, central inverter/charger and other DC equipment deemed lower priority, like LP Gas valves, washdown pumps, water makers, and most other pumps.  Sudden loss of this equipment is certainly, but not unsafe to crew and vessel.

Battery Charging

Just as lithium-chemistry batteries discharge differently from lead-acid batteries, they also charge differently; very differently.  It is quite common to see lead-acid and lithium batteries characterized as “dumb” vs “smart” batteries.  Perhaps readers have seen magazine or Internet articles suggesting:

    1. lead-acid batteries are “dumb batteries” that require “smart chargers,” and/or
    2. LFP batteries are “smart batteries” (because of the BMS) and can be charged by “dumb chargers.”

These are misleading battery characterizations, based on the presence of what is assumed to be an “intelligent” BMS.  The purpose of the BMS is to ensure the battery stays withing safe operating limits, not to “manage” battery charging.  In “drop in” batteries, a cheap Asian-made BMS may not trip off until an internal event triggering limit has already been exceeded.  Using the “disconnect capability” of the BMS to disconnect a battery from a charger when the battery reaches “full charge” is possible, but IS NOT good design practice on a boat, either for safety or for charge-discharge cycle lifespan.

Unlike lead-acid chemistry, partially discharged lithium chemistry batteries hold their maximum Charge Acceptance Rate (CAR) right up to the point of being fully charged.  A “dumb” AC (shore power or genset) battery charger doesn’t know or care when that fully charged point is imminent or actually reached.  That “dumb” charger just pumps amps into the battery.  In this scenario, the system designer is depending on the BMS to disconnect the battery from the charger, not on the charger to realize the battery is full.  The result is, when this battery reaches full charge (or over-charge), the BMS will disconnect the battery from its loads, possibly damaging the charger, and certainly leaving the charger itself as the only source of power for DC attached equipment.

After an overnight on the hook, the “house” battery bank is partially discharged.  The crew gets underway as usual.  The engine alternator has the task of recharging the bank.  As the morning hours wax towards noon, the battery bank accepts charge.  When the bank reaches full charge, the BMS disconnects it from the system.  That sudden disconnect event brings with it all of the attendant potential problems that have been previously discussed.

Lithium Chemistry batteries do not need to be “floated,” and some manufacturers say not to float them.  The issue is, a “float” voltage set too high risks over-charging an LFP battery, so the float voltage must be very carefully set to be compatible with the LFP manufacturer’s specifications.  But as an “electrical system” design consideration, a lead-acid battery charger in “float” mode has two functions.  One is to maintain the lead-acid battery against the effects of self-discharge and the other is to provide DC power to DC equipment.  In a lead-acid system with the battery charger in “float” mode, DC B+ power for loads comes from the battery charger, NOT the batteries.  With an LFP battery, the self-discharge mission has gone away, but the need to provide B+ power to house loads remains.

Best practice:

    1. lead-acid batteries are “best charged” by a multi-stage (“smart”) charger;
    2. LFP batteries are “best charged” by a constant-current capable charger sized to the charge current specification of the battery, and set to discontinue charging at less than 100% SOC;
    3. a modern lead-acid “smart charger” with both lead-acid and LFP charging programs will charge both battery chemistries just fine.
    4. LFP batteries should not be “floated,” lest they over-charge and trip their BMS; but if “floated,” they should be “floated” at or below the voltage recommended by their manufacturer.

Pro Tip: In a lithium chemistry system, it matters how the battery charging systems are designed.  It is unwise to let an engine alternator with a “dumb” internal voltage regulator drive the LFP battery bank to its absolute full charge.  Alternators should be fit with a “smart” external voltage regulator that can discontinue constant current charge when the LFP target bank is at 80% SOC instead of 100% SOC.

Pro Tip: Very careful design consideration MUST BE GIVEN to ANY SYSTEM where the BMS will be relied on to terminate charging.  In any system of that design, the LFP battery will be disconnected from it’s load, and that means the load may lose DC power.  In most boat applications, that IS NOT desirable.

Charging multiple banks:

With shore power (or genset) AC, it’s easy to install a battery charger, or multiple battery chargers, to charge multiple, different battery banks.  Charging with engine-driven alternators, which don’t lend themselves to being “stacked” in multiples onto an engine, requires more analysis.  Alternator configuration depends on the number of engines and presence of any available auxiliary alternator mounting locations.  With single engine boats, alternator output capacity is a paramount design consideration.  With multiple engines, additional charging options do present themselves.  Each owner must consider this requirement in the context of presently existing equipment.

Particularly on boats with multiple lead-acid battery banks, it is common to find Voltage Sensing Relays (VSRs) used to isolate existing lead-acid “start” and “house” battery banks from one another when not charging, and to connect them together for charging.  A VSR is just a dumb solenoid.  Current can flow in either direction between the two banks.  For a lead-acid “start” bank to be paralleled to a lead-acid “house” bank with a VSR, the “house” battery/bank should be permanently connected directly to the alternator.  The VSR would “parallel-in” the “start” battery/bank.

After an overnight on the hook, upon “engine start”, the alternator voltage regulator initiates “bulk mode” to bring the terminal voltage of a partially discharged “house” battery up to it’s 14.5V bulk target voltage.  As terminal voltage ramps up, the VSR senses the “connect” voltage and “parallels-in” the “start” bank.  At VSR-connect time, the “house” and “start” batteries are close to each other, and so the inrush current exchanged between the two battery banks through the VSR is low.  When the VSR solenoid is closed, the two battery banks are electrically in parallel and charge together, as one.

Because of high inrush currents, VSRs are less desirable for paralleling a lead-acid battery and an LFP battery, particularly in the case where the banks are NOT both fully charged.  In the case of a solenoid connecting a partially charged LFP “house” battery to a mostly charged lead-acid “start” battery, there will be a huge mismatch in the charge and equivalent resistances of the two, and there will be a huge inrush current to the partially discharged LFP “house” battery from the mostly charged lead-acid “start” battery.  That large inrush current looks like a starter motor load to the “start” battery.  It discharges the “start” battery (undesirable) and may look like a short circuit to the LFP BMS.  If so, it will cause the BMS to disconnect (undesirable).

Assuming the configuration survives the VSR-connect, the LFP BMS will disconnect its battery when any one of the internal cells reaches an over-voltage SOC.  In order to avoid that unwanted BMS disconnect, the above VSR configuration would require the charging source behind the battery pair to be “smart;” that is, “programmable,” so that the overall charging process can be stopped at a voltage preset low enough to avoid having the integrated BMS disconnect the battery.  That also helps protect the alternator from overload and over-heating and from throwing a voltage spike at BMS Disconnect time.  That can be accomplished with an external, programmable voltage regulator fit to the alternator.

Pro Tip: For interconnecting lead-acid and lithium chemistry batteries at different states-of-charge, consider use of DC-to-DC Converters in place of VSRs.  Logically, these two devices are equivalent; both “parallel-in” another battery bank for charging and both isolate banks from one another when not charging.  But unlike “on” or “off” VSRs, DC-to-DC converters isolate the two battery banks and pass current in only one direction.  Depending on features, DC-to-DC converters can appear to their load side as “smart chargers,” controlling the voltage at the load and limiting the current supplied to the load.  The DC-to-DC converter can accept either “smart” or “dumb” input sources.  Upgrading the system from VSRs to DC-to-DC Converters is a recommended conversion-cost in switching to LFP batteries.

Solar Systems:

This article is already quite long, so I will just note here that I haven’t forgotten solar and wind charging sources, but have intentionally omitted including them in this discussion.  The “electrical system” compatibility concepts are all the same.

Conversion Costs:

The obvious initial consideration of conversion costs from lead-acid to LFP batteries is the cost of the batteries themselves.  LFP batteries can be discharged to a larger percentage of capacity than lead-acid batteries.  Thus, if 450 aHr of usable energy is needed to support a boat’s cruising needs, ~1000 aHr of lead-acid batteries would be needed to get that capacity, but only ~500 aHr of LFP would be needed to get the same usable capacity.  The lead-acid solution prices out to less than $1000.  The LFP solution prices to 3x – 5x that of lead-acid; there are highly variable unit costs depending on the quality of the LFP cells being purchased.   In this calculus, assumed service life projection (charge/discharge cycles) appear to drive the value proposition of conversion positive.

As discussed above, there are other potential costs related to conversion.  Every boat is unique.  The more existing batteries and battery banks there are aboard, the more charging equipment and monitoring complexity comes with conversion to lithium chemistry power.

This is particularly true if there are different battery banks of different voltages (mixed 12V and 24V) that require different charging equipment.  Take a moment now to carefully review Figures 1 – 3.  Identify the figure that most closely approximates what you have on the boat you currently own.  Figure 5 shows a system with both 12V and 24V house battery banks.

These drawings should help to visualize the scope of an actual conversion project.  With them, identify what areas might need reconfiguration, and begin to consider the steps that might be involved in a conversion project plan.

For Sanctuary, re-wiring branch circuits to separately power “high priority,” “medium priority” and “low priority” branch circuits would be very challenging given the OEM cabinet and bulkhead arrangement and the locations of the existing batteries, OEM electrical panels, OEM cabling and existing installed equipment.  Without major interior remodeling, batteries would have to remain located in the engine room, where cooling the batteries is already problematic and additional cooling would definitely be necessary.  Addition of new B+ power buss would require at least a revision to the power panel configuration.  New cabling would need to be installed in in already crowded wire chases and raceways.  Conversion to a technology that is more sensitive to ambient temperatures and involves significant remodeling of the living space on the boat is unjustifiable, both economically and in terms of operating risk.  Every owner has to look at this from their own, unique perspective.  Conversion and reconfiguration cost to obtain “a reliable electrical system that is safe for crew and vessel” is unique to each vessel and the needs and desires of each owner, but will not be insignificant.  Read that as, more than expected and more than planned.

Considering all of the preceding, I’ll again ask the boat owner who is considering lithium chemistry batteries the following questions:

    1. How do you use your boat?  Do you cruise from marina-to-marina?  If you have a “shore power lifestyle,” do expensive batteries and complex “electrical systems” still have value?
    2. Do you live aboard, enjoy continuous cruising, endure a short boating season, encounter long idle periods, lengthy summer/winter layup?  Idle periods and layup periods require different planning for LFP batteries than lead-acid batteries; cold weather vs hot weather layup requirements are different.
    3. How long have you owned your boat, and how long do you intend to own it?  Financial ROI for lithium chemistry batteries is 3 – 4 lifecycle turnovers of lead-acid batteries (minimum of 10 – 12 years), much longer if initial “electrical system” conversion costs are included in the calculus.
    4. What’s the boat’s present age and present market value?  Conversion value will be lost if you should decide to sell in the near-term future.  It’s unlikely conversion to lithium would add market value to the boat.
    5. Are there insurance or insurability implications to conversion to lithium chemistry?  LFP is a very stable chemistry, but if it does burn, there’s no extinguishing it.  A lithium fire is an “abandon ship” event.
    6. Why is it worth it to you to convert to lithium chemistry batteries?  What are the value benefits to you of having lithium chemistry batteries?  Will a conversion of battery chemistry fix something that isn’t working in the existing electrical system on your boat?  Are you sure?
    7. Given the rapid present evolution of battery, system and component technology, does the boat owner have the skills, resources and will to repetitively redesign and upgrade the electrical system?  Does the owner have the skills to keep the system current with improvements in equipment?  DIY electrical skills have huge Net Present Value (NPV) in the conversion calculus.  Commercially hired, contracted electrical skills in 3Q2021 will price out at $140/hour, ±, plus equipment/materials markup.  Often, the labor cost component will exceed material and equipment costs.
    8. Does the owner of a cruising boat have the DIY skills to troubleshoot electrical system issues that occur “while cruising in exotic places?”  If not, when something fails, “where in the world” (literally) will the boat be when a failure occurs?  Will parts, replacement equipment and technician skills be available when and where they are needed?
    9. Does the boat owner have temperament and tolerance for possible “system outages” and cruising delays?   Spouse also temperamentally suited to outages?  What anxiety levels are OK, and when do anxieties become overwhelming?

At this point in this article, we have considered:

    1. how the owner uses the boat, in cruising and in layup,
    2. how much longer the owner plans to own the boat,
    3. analysis of the existing electrical system in place aboard the boat,
    4. prioritization of the safety importance of DC circuits aboard the boat
    5. analysis of owner electrical skills and conversion costs,
    6. characteristics, strengths, weaknesses and maturity of current LFP solutions, and
    7. owner tolerance for outages, including how outages might impact upon our key crew members.

Matching Batteries to Applications:

Start Service:” Battery C-Ratings must be matched to starter motor current draw magnitude and draw duration.  It’s not enough to limit capacity considerations to starting a properly adjusted, properly tuned, properly running engine under ordinary, normal conditions.  Consideration must be given to the cranking needs of the engine under in-service and post-service conditions, where prolonged periods of cranking load will occur.  Except with high C-Rated LFP batteries controlled by a single, robust BMS, lead-acid batteries are best suited to “start” service applications, including windlass, winches and thrusters.  Groups of “drop-in” LFP batteries, each individual with its own BMS, may not be well suited to “start service” applications.

House Service:” DC “House” loads judged to be “high priority” need to be matched with highly reliable battery sources.  This is an application where hybrid banks of mixed lead-acid and LFP components are well-suited, and were discussed in this linked article.   The LFP component of the hybrid provides the benefits of voltage stability to loads, while the lead-acid component of the hybrid provides the rarely needed but critically important alternator protection, continuous availability and high reliability in the event of sudden BMS disconnect.  “Low priority” house loads where intermittent transients are few, and amp draws are modest, are well suited to LFP banks without the lead-acid component.  One fully integrated, 400 aHr C-Rated LFP battery with robust BMS is a better option than four 100 aHr “drop-in” batteries, each with separate BMS, arranged in parallel.

Inverter Service:” “Inverter” loads are generally not mission-critical loads.  Inverter banks do have some transient high amp demand characteristics which need quantification.  Some AC loads create very high, short term transient demands in DC battery loading.   A bank with a microwave, coffee maker, toaster oven or induction cooktop will create dramatic transient loading fluctuations in normal use, and the C-Rating of the inverter bank must be capable of handling the maximum transient load for its entire duration.  If cooking pasta in a microwave takes 28 minutes at 50% microwave power, the BMS over-current setting must be able to support the corresponding battery DC amp draw duty cycle for its full duration.  As a design choice, doubling the bank voltage (12V → 24V) cuts amp demand in half, but simultaneously obsoletes 12V equipment, necessitating upgrades, and so adds to conversion cost.

Commercially available “system” solutions:

I do not feel LFP-based platforms are, as yet, “install and forget” solutions suitable to a general consumer boating market or owners without at least moderate electrical technical background.  Today in 3Q2021, the hardware solutions being rolled out by leading equipment manufacturers in support of lithium chemistry applications are in varying stages of technical maturity.  The majority of today’s buyers enjoy DIY projects, are technically proficient (because they have to be), and fall into the category of “early technology adopters.”  Reliable technical advice is not yet widely available.  Battery and equipment manufacturers generally understand the state-of-maturity of their products, and are working hard to close system reliability gaps.  New equipment versions are rolling out in cycles of deployment of 6 – 9 months per manufacturer.  Expect rapid hardware evolutions and rapid product obsolescence over the next several years.  Also expect that at least some manufacturer names that are well known in 2021 may not be here at all in 10 years, putting warranty promises in doubt.

Summary:

A lifetime of engineering project development experience suggests that as insights are gained from early adopters and system failures, incremental improvements will be staged into existing products and net new, increasingly feature rich products will become available.  User options will expand, efficiencies will improve, and costs (capital, conversion and maintenance) will come down.  That is where I believe we are today, in what is essentially a proof-of-concept and beta test period with the rollout of lithium chemistry batteries and system solutions to informed early adopters.  There remain important, unanswered design and safety questions.  It’s quite likely that the equipment beings sold today as “leading edge” and “ground breaking” will be viewed as “primitive” and “inadequate” in 7 – 10 years, so should be considered by buyers as “interim solutions.”  Given the very long ROI timeframe of a conversion project, it’s likely batteries and systems will be far advanced well before cost recovery for equipment installed today has been realized.  There’s not much room for manufacturers to obtain ROI on product investment in this small and expensive market space, so expect high prices and availability constraints for the best quality equipment.

Today, lithium chemistry battery solutions do have a small market for which they are well suited: that is, folks who meet all of the following criteria.

    1. DIYers possessed of “advanced” or “expert” electrical skills,
    2. living-aboard boats that remain in continuous service year-’round,
    3. fit with large solar systems (800W or greater), and
    4. cruising in remote and “off-grid” places, away from shore power.

That profile does not describe a very large cross-section of recreational vessels or recreational vessel owners.  For most of us, lead-acid batteries remain the simplest, best and most cost-effective battery solution; granted, not glitzy, but serviceable, utilitarian and affordable.

There is a very aggressive research and development effort going on in the lead-acid sector, and if it realizes anywhere near its potential promise, will change existing technology and financial equations yet again.  (Well, did you really think lead-acid interests would go off into the corner and die?)  It is unlikely that the currently available lithium chemistry batteries will live up to their “hype” in the real world.  Lithium chemistry batteries are not going into boats with a Net Present Value below about $250K.   Presumably that means they’re going to people who can both afford the conversion expense and absorb the hidden costs of failed or inadequate conversion assumptions and design decisions.

In the medium term, those boaters enduring “major lithium chemistry envy” should consider carbon foam lead-acid batteries.  Carbon foam may offer a very acceptable incremental compromise toward the overall advantages of lithium-chemistry, but without the BMS-related risks.

This article has discussed LFP batteries in the context of the “electrical systems” in which these batteries function.  It has not discussed the technologies of construction of the battery cells, BMS electronics or BMS design shortcomings, electrical safety components not yet available in a lithium platform system or the economics of the current manufacturing infrastructure.  It’s fair to say this discussion “glosses over” much of the technical detail related to the batteries themselves.  But, hey!  Boat owners don’t want to have to know that stuff anyway, right?!  I refer those interested in a deeper dive into the technical detail to Rod Collins’ wonderful website for his excellent article entitled, “LiFePO4 Batteries On Boats.”

Lithium Chemistry Batteries on Boats

Initial Post: April 22, 2021

Topics on “Lithium Ion Batteries” fill textbooks, but I know there is great interest in the boating community.  This article is intended as a “brief” survey of the possibilities and challenges associated with Lithium chemistry batteries.  My focus throughout is on “electrical system reliability.”  The length of this article grew very quickly, so I’ve limited my content to areas that often lack adequate consideration.  “How do I make this simple for readers to understand?”  That is my primary goal and has been a real struggle.  To limit length, I’ve had to assume that readers are already somewhat familiar with the advantages, operating characteristics and risks associated with lithium chemistry batteries.  I think that describes boaters actually contemplating adoption of lithium batteries.

A reliable electrical system is essential to crew and vessel safety on any boat.  Lithium chemistry battery articles typically focus on lithium technology and its benefits.  Such articles rarely discuss the implications to the overall reliability of the vessel’s electrical system.  Modern boats are often in immediate peril (physical risk, operator anxiety, inconvenience, lost time, and cost) if any part of the vessel’s electrical system incurs a fault or outage.  This article considers the possible – not “inevitable,” perhaps “infrequent,” but certainly “possible” – reliability impacts of lithium chemistry batteries to the “Electrical System” of boats involved in cruising.

Lithium chemistry battery systems involve more technical complexity than their lead-acid cousins.  Lithium chemistry requires a “control system” to keep the battery’s internal electro-chemistry within limits that are safe on boats.  Design details required of these control systems are maturing rapidly, but in 1Q2021, new requirements are still emerging from the good and bad experience of early adopters and the experience gained from early system implementations rolled out by leading equipment manufacturers.  Regarding lithium chemistry batteries, we cruisers are in a period best described as one huge “beta test.”  Some designs are proving valuable and others are proving unsuitable.

The Boat “Electrical System”

I have long “preached” that boaters need to think about boat electrical systems “as an integrated system” and not as “individual components” hooked together.  Engines with DC fuel solenoids/electronic fuel injection, alternator(s), start batteries, house batteries, battery charger(s), inverter(s), water and waste pump(s), bilge pumps, windlass, thrusters, windshield wipers, horns, DC nav equipment, nav lights, solar panels, DC generators, AC generators, hot water heaters, HVAC equipment, raw-water circulator pumps, watermakers, coffee makers, space lighting, washer/dryers, refrigerators, microwaves and other galley equipment, SOHO routers, computers, entertainment electronics, video cameras, boat monitors; all are “components” of the “electrical system.”  While each has unique needs, reliability means all must work cooperatively together, as a whole; never has that been more true than with lithium technology batteries.

Figure 1 is a simplified drawing of a “typical” cruiser’s Electrical System.  The AC sub-division is shown in green, and the DC sub-division in red.  The interface layer between the AC and DC sub-divisions, in which power flow is “bi-directional,” is blue.  This is a “direction-of-power-flow” diagram (source-to-load), not a wiring diagram.  In this “model,” the DC sub-division emerges as the “soul” of the boat’s overall electrical system, and the house battery bank is the “beating heart” of the DC sub-division.  Most of the “stuff” that comprises the AC sub-division is “optional;” i.e., discretionary to the lifestyle owners want to live while aboard.  The “stuff” that comprises the DC sub-division isn’t optional; without that “stuff,” we don’t have a safe, operational vessel.

Above, the DC sub-division “battery bank,” as shown, can be viewed as either 1) a “central” bank that “does everything,” including engine starting, or 2) a “House” bank with an auxiliary “Start” bank “hung off the side.”   Both views have at least two battery charging sources: 1) an AC-powered charging source and 2) an engine alternator.  There are multiple approaches to battery charging, and many different combinations of charging equipment.

Batteries provide DC power to all of the vessel’s DC loads.  Some boats are set up to have a third, entirely separate and independent bank that supports the inverter and its AC loads.  However battery charging is done, in the end, all battery banks must be charged.  The more banks, the more equipment and monitoring complexity, and maintenance, is involved in managing charging.  The current design platform (Unique Figure 1 configuration) of a boat’s “electrical system” will have enormous impacts on how to best adopt a suitable, compatible lithium chemistry solution.

Lithium “Energy” Cells

Lithium battery material is a complex “chemical soup;” in this article, 3.2V cells of “Lithium-Iron-Phosphate,” or “Lithium Ferro-Phosphate,” or “LiFePO4,” or just the “LFP” chemical compound.  This lithium technology is the safest of the lithium technologies, but it’s not as safe as lead-acid.

Figure 2 shows the two principle ways that commercial lithium cells are packaged; i.e., “prismatic” and “cylindrical” construction.  The word “cell” refers to any package with its own positive and negative electrical terminals.  Cylindrical cells are cylindrical in shape, within their own case.  Cylinders are a physically strong shape and are relatively easy to temperature regulate.  Prismatic cells are composed of thin layers of “soup” folded and sandwiched together and packaged into an aluminum or steel container.  Prismatic cells are easy to handle and take less space than packs of cylindrical construction of the same aHr capacity.  In packs made up of cylindrical cells, if one cell fails, the pack will continue its useful service, albeit at a lesser reduced capacity.  If one cell inside a prismatic package fails, that failed cell may “take out” sibling cells and the entire prismatic package may fail.  Cylindrical cells are less expensive to manufacture and are more durable than prismatic cells in some applications.  In automotive, RV and marine applications, where the platform is subject to constant mechanical shock and vibration, cylindrical cells are more durable than prismatic cells.  Prismatic cells can be utilized effectively in these applications, but measures must be taken to mitigate and absorb mechanical shocks and vibration.

Battery Terminal Voltage and Amp Hour (aHr) Capacity

Figure 3 shows that the more chemical “soup” in a pack of cells, by volume, the more energy that pack can store: i.e., “physically bigger pack, more Amp Hour (aHr) capacity.”   LiFePO4 “soup” contains 220 Wh/L of energy storage capacity, so liters equate to stored energy.   In describing battery construction, the battery industry describes the number of cells-in-series, followed by the number of cells-in-parallel; i.e., “2S2P.”  Four 3.2V packs stacked in series (4S) yield a nominal “12V” battery.  Packs can be stacked in series to create 12V, 24V, 48V, and greater nominal voltages, suitable for use in a wide range of applications.

Four 3.2V, 100 aHr cylindrical-cell packs (or prismatic cells) stacked in series (4S) makes a 12V battery of 100 aHr capacity.  Eight 3.2V cylindrical-cell packs (or prismatic cells) configured with two in parallel (2P) and four pairs in series (4S) results in a 12V battery of 200 aHr (4S2P).  So, in a 4S4P configuration of 100 aHr cells, the result would be a 12V battery of 400 aHr capacity, and in a 4S6P configuration of 100 aHr cells, the result would be a 12V battery of 600 aHr capacity.

Lithium Chemistry Batteries

Compared to lead-acid chemistry batteries, lithium chemistry batteries are:

  1. more volatile in response to internal temperatures, so are:
    • extremely sensitive to high and low ambient temperature conditions,
    • more sensitive to rate-of-charge and rate-of-discharge,
    • more sensitive to “thermal runaway” events;
  2. more susceptible to permanent damage caused by a very deep state-of-discharge;
  3. more subject to permanent damage from a “short circuit” accident;
  4. more subject to voltage drift between cell-packs of the battery, so
    • subject to both High Voltage Events (HVE) and Low Voltage Events (LVE);
  5. have unique/different charge and discharge characteristics and needs, and are
  6. more susceptible to physical shock and vibration

Figure 4 depicts the construction of a completed lithium chemistry “battery,” shown surrounded by the heavy blue border, box-labeled “Battery Assembly.”

All Lithium Ion batteries need a BATTERY MANAGEMENT SYSTEM (BMS) to manage their electrochemical behavior.  The BMS is a package of electronic sensors and circuits that ensure the chemical cells remain within safe operating limits.  The BMS monitors charge and discharge currents, internal temperature, and voltage variations between packs. The BMS manages voltage equalization between packs.  With lithium batteries, owners must be aware of what the BMS is doing, and why it’s needed.

The battery “assembly” shown in Figure 4 is representative of a single, large-capacity, lithium chemistry battery composed of four prismatic cells.  This option is practical for cruising boats in 2021.  A single 600 aHr capacity lithium battery (cylindrical or prismatic) is approximately equivalent to the usable capacity of a 1000 aHr lead-acid battery bank.  Prismatic 3.2V cells are widely-available, commercially-manufactured components.  The BMS is available as a commercially-manufactured electronic component, which optionally may come equipped with an Alarm Warning feature.  The disconnect solenoid (the BMS Disconnect switch) is a commercially-available component.  The battery “case” must provide mechanical protection to the packs, but can be as simple as a plastic milk carton with closed-cell foam cushioning or range to a custom-made fiberglass container.

This overall “solution” amounts to a “kit.” (Yea, “Heathkit!”) The kit’s designer (and general contractor) must specify the bill-of-materials, source the components, order them and install them.  The kit installer can be a boat builder, boatyard mechanic, independent consultant or DIY owner.

Pro Tip: When sizing batteries to match the “electrical system” power requirements of a boat, remember that both lead-acid and lithium chemistry batteries lose aHr capacity as they age.  That loss can be as much as 20% of the rated capacity of the batteries (0.2C).  Plan accordingly.

Balancing the Cell Packs

In the “kit” described above, nominal 3.2V packs are made up of paralleled cells.  “New” packs from the supplier can be at a different SOC, so the actual pack voltage can range anywhere along the nominal LFP voltage curve.  Pack voltages must be “balanced” before being placed in service, to equalize voltages variations.  Packs can be either “Top Balanced” or “Bottom Balanced.”

Figure 5 shows the open-circuit voltage curve of a 3.2V lithium chemistry cell at different SOCs.  LFP cell voltage remains very flat from maximum-charge to maximum-discharge, and Peukert’s Factor has little effect at high levels of discharge current.  For the “kit” builder, the “out-of-the-box” task is measuring the terminal voltage of each pack when delivered.  New cells should be nearly the same voltage; ex: 3.2V ± 100mV.

Because the voltage discharge curve of Lithium chemistry cells is very “flat” from “full” to “empty,” any “brand new pack” could be at 70% SOC and any other “brand new pack” at almost the same voltage could be at 30% SOC.   Early in “kit assembly,” the individual packs would not yet have a BMS to protect them.  The assembly-time risk is permanent damage to a brand new pack by accidental over-charging or over-discharging.  This IS NOT a warranty claim; manufacturer’s consider it “abuse.”  It’s a very costly “user error.”

Figure 6 is another view of the pack-to-pack voltage variation, with four nominal 3.2V cells at slightly different SOC voltages.  Wired in series while charging or discharging, these unevenly balanced pack voltages would rise and fall by the same proportional amounts, in response to the current flowing through them.  The relative voltage differences would remain as pack SOC increased or decreased.  Charging, when the lowest voltage cell reached full charge, the three higher voltage cells would be in the over-voltage danger zone (overheating, O2 outgassing).  Discharging, when the highest voltage cell reached its discharge low-voltage limit, the lower three cells would be in the under-voltage danger area (cathode disintegration).  To avoid over-voltage or under-voltage cell damage, “kit build” packs must be “balanced” during assembly.  In consideration of the length of this article, I’ll send readers interested in the details of the balancing process to Youtube.  This is a link to a Youtube video I think explains balancing quite well (Russian accent excepted).

The “Dark Ship” Moment

Lithium chemistry batteries are all fit with a BMS, which is intended to protect the battery.  If the BMS senses a high or low temperature, a High Voltage Event (HVE), a Low Voltage Event (LVE) or an over-current event, it should ideally emit an Alarm Warning Signal.  After a brief delay, the BMS will disconnect the battery from the host DC system.  Reliability of the host “electrical system” is critically important!  The BMS is there to protect the battery; it does not consider potential adverse impacts to the host vessel in its disconnect decisions.  Depending on the configuration of the individual boat’s DC electrical sub-division, a BMS Disconnect can be an “everything just went dark” moment.  Knowing the impacts of a BMS Disconnect event to the host DC “electrical system” is the responsibility of the system designer (and boat owner).

Figure 7 depicts the situation where the BMS detected a safety threat to the battery.  The BMS initiated a BMS Disconnect event.  The Disconnect Solenoid is an internal part of the battery assembly.  When it opens, everything powered by the battery simultaneously loses DC power.  Many engines will shut down, and their instrument cluster lights and gauges fail!  The RADAR, Chart Plotter, Autopilot, Depth Sounder, VHF, SSB, AIS, electronic throttle and steering controls (fly-by-wire) all fail.  The horn, foghorn, windshield wipers and inverter are in-op!  Space lighting failed.  The silence is deafening!  The lithium chemistry battery pack is safely protected, but the ship is dead in the water.  The rudder position is whatever the rudder position was.  “Dark Ship” events are possible, not inevitable!  They don’t happen often, but they can and do happen.  Good “Electrical System” design is what moderates/prevents them.

Pro Tip: A separate, stand-alone lead-acid bank that provides DC for critical Nav equipment can moderate the “Dark Ship” impact, but also adds complexity to the vessel “electrical system.”

Lithium chemistry batteries made up of ganged prismatic 3.2V packs, including BMS, are usually “repairable” after a failure via parts-replacement.

Pro Tip: “cheap” is not a good word to describe a BMS.  Cheap electronic parts are cheap for a reason; they’re made of cheap components.  A BMS failure is a big deal.  Buy for quality!  The longer the warranty, the better.  Remember, the BMS is internal to a “drop-in” battery’s case.

“Drop-in” Lithium Chemistry Batteries

Figure 8 shows three individual “drop-in” lithium chemistry battery assemblies wired in parallel to provide DC power to the boat.  These “Drop-in” batteries are composed of cylindrical cells made up into 3.2V packs.

This paralleled LFP configuration is analogous to multiple, paralleled, deep cycle lead-acid batteries.  Today (1Q2021), commercially-available 12V “drop-in lithium chemistry solutions,” are available in the market.  “Drop-in” batteries are made by several companies, in the size and shape (“form factor”) of Group 27, Group 31, and even 8D 12V lead-acid batteries.

Assuming each of these lithium chemistry “drop-in” batteries has a rated capacity of 200 aHr, this bank would have a total capacity of 600 aHr, equivalent to the earlier prismatic example.  If one BMS detects a fault, that individual battery would drop itself offline.  Depending on the cause of the fault, it may or may not be possible to return that battery assembly to service.  The individual components of “drop-in” batteries are not separately serviceable.  If internal BMS electronics or other internal battery component fails, that battery assembly would become a toxic waste “brick.”

In preparing this article, several manufacturer websites were queried for information on currently-available “drop-in” batteries and system solutions.  Some highlights:

      • VictronEnergy® seems by far the most “thorough and complete” of all the manufacturers in lithium battery SYSTEM technology.  They provide a complete solution, including the batteries, BMS, monitoring components and battery charging options.  In this solution, several monitoring system components are widely distributed throughout the host “electrical system.”  These components were not previously necessary in lead-acid systems.  Victron® appears compliant with the current content of the ABYC TE-13 Technical Report, and their BMS does provide a pre-alarm prior to BMS shutdown.  The pre-alarm is utilized by their own add-on components via a proprietary networking arrangement.   The possibility of a “dark ship” event is not eliminated in this system’s solution design.  This solution is rolled out as individual parts and is currently in “beta test.”
      • Battle Born® (DragonFlyTM) is not TE-13 compliant at this time (1Q2021).  They provide only the “drop-in” batteries, and not system-wide monitoring equipment or solutions where multiple batteries would be co-resident.  They point users at Victron® accessory components in some areas, but for monitoring, not automatic control of their “drop-in” units.  Controls are still manual, the owner/operator’s responsibility.  They say they are working towards compliance “in the near future.”
      • Xantrex® seems to have at least a visual alarm on the “drop-in” battery case, consisting of rapidly blinking LEDs.  They have an external monitor that can be attached to multiple Xantrex batteries (doesn’t say how many, but at least two).  There is no indication that a battery warning from one battery is available for use in externally connected equipment or that it can be utilized by non-Xantrex equipment.
      • Renogy® does not mention any type of warning alarm in advance of a BMS Disconnect. These batteries cannot be connected together in series (two 12V batteries in series to create a 24V system).

Protecting Alternators

For the designer/installer/owner of a lithium chemistry battery, it’s important to realize that one of the most essential components in the entire “electrical system” is the engine’s alternator(s).  Alternators do their work through the magic of magnetic fields.  The field winding of the alternator is a mechanically-driven, spinning electromagnet.  The electromagnet gets its electrical strength from a voltage regulator.  As the electromagnet spins, it induces an electric current in the alternator’s stator windings.  The induced 3-phase AC voltage is converted to DC output by an internal solid-state rectifier diode pack.  The primary electrical load on a working alternator is the battery/bank that the alternator is charging.  LFP battery load (charging current) can be very large, so the magnetic field generated in the stator winding is very significant.

Pro Tip: it is always a good idea to install alternators fit with external voltage regulators on boats.  This simple configuration alternative is appropriate to both lead-acid and lithium chemistry batteries.  External regulators have user-selectable, technology-specific, multi-stage charging programs, and are able to monitor alternator case temperatures.  In response to high case temperature, the voltage regulator modulates field drive to protect the alternator from overheating.

Lithium chemistry batteries accept very large charging currents, so “electrical system” designers assume that alternators will be working towards their maximum capacity throughout most of the battery-charging cycle.  Lithium chemistry batteries charge in “Bulk” mode to 99% SOC.

Pro Tip: charging programs for shore power chargers, engine alternator voltage regulators and solar controllers must be set so the maximum B+ voltage stays beneath the HVE voltage disconnect specification for attached BMSs.  Lithium chemistry (LFP) has a High Voltage Cutoff of ~14.6V.  An HVE can occur because of internal pack drift (pack-to-pack voltage variations) or it can be caused by an externally impressed charging voltage coming from an external charging source.  Obviously, the charging source itself must not become the cause an HVE disconnect.

Alternators must never be disconnected from their load while they are producing output.  Sudden loss of the battery connection to an alternator is usually catastrophic to the alternator.  A BMS Disconnect event is exactly that: a “sudden alternator disconnect” in which the alternator output current is suddenly interrupted.  This results in the sudden collapse of the magnetic field in the alternator’s stator windings.  The collapsing magnetic field results in a huge voltage spike in the alternator windings.  Disconnected from its load by the now open BMS Disconnect solenoid, there is no place for the spike energy to go to be safely absorbed.  That spike voltage either destroys the alternator’s rectifier diodes or damages the insulation of the alternator’s stator windings, or both.  And that’s not all of the bad things that can happen!

In a typical boat “electrical system,” the alternator is one of many device attachments to the B+ buss fed by the battery/bank.  Also attached to the B+ buss is all of the rest of the boat’s DC equipment which was simultaneously in use at the moment of BMS Disconnect.  Some of that equipment is “delicate electronic equipment” (RADAR, Chart Plotter, Autopilot, depth sounder, VHF, AIS, inverter, solar controller, etc).  The spike that kills the alternator is also impressed onto the boat’s B+ Buss, so all equipment attached to that buss is also at risk of damage.

Pro Tip: it is always good to install a surge bypassing diode at any alternator on any boat.  Surge Protective Devices (SPD), also called Transient Voltage Suppressors (TVS), are used to shunt high voltage spike energy to ground, thus also protecting equipment connected to the buss.  While “good” to do, this strategy is not a 100% reliable damage mitigation solution.  Depending on a range of variables, the spike voltage can exceed the capacity of commercial DC SPD products.  SPDs that do successfully suppress transient spikes can be left in a partially or totally “damaged” state by the event, and may then not function correctly, or at all, in a future event.  So this strategy, while useful, is not “sufficient” to the overall goal of protecting alternators.

Figure 9 is a simplified view of a typical alternator application which includes an SPD.  The alternator field coil gets its “exciting” DC voltage from a voltage regulator, shown here as external to the alternator case.

A BMS disconnect scenario in this large-capacity, single battery bank would start with a problem detected by the BMS.  The BMS would send out an alarm signal, indicating an imminently pending disconnect event.  Good system design would then use that warning alarm signal to shut down the alternator’s field drive by shutting down the voltage regulator.  That design removes field drive from the alternator, so it shuts down gracefully, without damage to the alternator, the protective SPD or any sensitive buss-attached equipment.

After a momentary delay, the BMS Disconnect solenoid would open, and the vessel would sustain a “Dark Ship” event.  Bad enough, true, but at least the alternator and electronics attached to the B+ buss wouldn’t be damaged as a result of the disconnect event.

A key assumption in this scenario is that the BMS has the alarm warning feature.  Not all BMS designs have that capability, so the DIYer specifying a BMS for a “kit build” has to know its available, needed and, indeed, essential to the integrity of the “electrical system.

Solar Controllers

The preceding discussion, “Protecting Alternators,” applies equally to “electrical systems” containing solar controllers.  Solar controllers receive a high and moment-to-moment variable input voltage from solar panels and convert that input voltage to “battery charging voltage.”  In doing so, they act as chargers for attached batteries.  A sudden BMS Disconnect disconnects the battery from the B+ buss, but leaves all other DC equipment attached.  If the battery – in this case, the load on a charging source –  is suddenly disconnected, solar controllers can be damaged in a manner similar to the way alternators are damaged.  This can happen in two ways.  One, by an alternator spike event.  But also, by sudden voltage imbalances within the controller caused by the sudden loss of load.  Mitigation of this potential problem is important and needs to be considered in the overall design of the “electrical system.”  This is another case where a BMS Alarm Warning system can be used to gracefully shut solar controllers down without damage.

Pro Tip: in the solar industry, the guideline is to cycle lithium batteries between 20% and 80% SOC to improve battery longevity.  That is also a useful strategy on boats, which carries a potentially huge hidden benefit.  When charging less than max-high, and discharging less than max-low limits, it’s much less likely to encounter an HVE or LVE.  Even if one of the packs of the battery (or one of a “drop-in” set) is, or has become, unevenly balanced, its much less likely that condition will result in a HVC or LVC BMS Disconnect.  With an adjustable voltage charger, for 12V lithium chemistry batteries, select max-charge and max-discharge voltages to cycle the battery (battery bank) between 20% and 80%  (This is also suitable for boats in long-term layup.)

Electrical System Reliability With “Drop-in” Lithium Chemistry Batteries

Figure 10 shows three individual lithium chemistry “drop-in” batteries wired in parallel to form a single bank of suitable capacity to power a cruiser’s boat.  This configuration also carries the need to protect the alternator.  This multiple-battery configuration may not be able to meet that goal.  It does not have the means to shut down the alternator in a graceful and protective way, and there are other concerns with this approach as well.

Individual “drop-in” batteries have their own internal BMS, but today, only Victron Energy® has a “whole system” management solution that controls an entire battery bank of multiple “drop-in” batteries.  The “marketplace” for “drop-ins” is “all over the place” in design maturity.  Promotional literature is sometimes/often misleading.  Buyers MUST do technical “due diligence!”

Considering “system reliability:”

In a 12V system configured as shown in Figure 1, both nav equipment and inverter are powered from the same battery bank.  With LFP “drop-ins,” wholesale replacement of lead-acid batteries reprises the “Dark Ship” scenario.  Moderating that scenario requires DC sub-system reconfiguration prior to conversion.

In a deeply-discharged 12V configuration, one disconnected “drop-in” battery will increase DC load on its peers and can cause others to disconnect in a cascade-type scenario.  Would the owner/operator know that just one paralleled battery had dropped out?  How?  At a minimum, the bank would lose aHr capacity.  What is the system reliability impact of reduced aHr capacity to the system, even over a short time?  Many uncertainties and design concerns.

With “drop-in” batteries, the concept of “voltage balancing” still applies.  Consider a 24V system with 12V “drop-in” batteries in series.  New “drop-in” batteries can vary in SOC.  What’s going to happen if series batteries are slightly uneven, or slightly under-capacity for the installation?  If just one of several “drop-in” batteries in any configuration were to suffer a BMS disconnect event, there would be some impact to the host “electrical system.”

Cruising boats do not have uncomplicated, fast, simple escape and assistance scenarios.  Boat needs must be evaluated based on the nature of boats operating underway, at some distance from land.  Today, there aren’t good solution designs for the use of multiple, series-parallel “drop-in” batteries in a large capacity bank that meet the reliability needs of the “electrical system” on cruising boats.  The impact to the host “electrical system” of any one individual battery having a BMS disconnect is indeterminate, unpredictable, unique to each boat’s “as-is” wiring configuration.  BMS Disconnect events always come associated with risks.

An Alternative Way To Protect the Electrical System

Figure 11 shows a battery bank with a lead-acid chemistry battery wired permanently in parallel with one or more lithium chemistry batteries.   These two dissimilar chemistry batteries become one, single bank, charged and discharged as one.  They are NEVER separated in service (except for periodic maintenance activity where they need to be physically removed from service).  Operationally, they are the right and left hands of the same individual organism.

Good “electrical system” design is that the lead-acid battery remains connected to the B+ buss following any BMS Disconnect to “absorb” any voltage spike.  This design actually results in several significant “system reliability” benefits:

    1. The larger the capacity rating of the lead-acid sibling, the better spike energy would be absorbed to mitigate other severe side effects.
    2. Even if an attached SPD were driven into conduction, the chances of it surviving to fight another day are improved by the absorption behavior of the lead-acid sibling.
    3. With a permanently paralleled lead-acid battery as part of the bank, there would be at least some available “DC reserve power” to seamlessly continue powering the vessel’s engine electronics, nav electronics, fly-by-wire controls, lighting and safety equipment following a BMS Disconnect event.  That clear benefit does mitigate the negative consequences of a total “Dark Ship” event.

     

    Pro Tip: The “hybrid” schema shown in Figure 11 can be used with either a single, large-capacity lithium chemistry battery or with a battery bank made up of multiple lithium chemistry “drop-in” batteries arranged in parallel.  If the boat already has serviceable lead-acid batteries, keep them.  Simply install lithium “drop-in” batteries in parallel with the existing lead-acid set.

    How A “Hybrid” Battery Bank Works

    Note:  Later in this discussion, readers are referred to a Youtube video further describing this “hybrid solution.”  To align the two discussions, I’m using “typical” voltages as used in the video; i.e., the “equilibrium state” of a normally charged LFP produces a terminal voltage of ~13.2V, and the “equilibrium state” of a normally charged PbSO4 produces a terminal voltage of ~12.6V.

    The “natural resting voltage” of a lead-acid battery is measured 24 hours after the battery has been fully charged and fully disconnected from all load circuits and from its siblings in a bank (four hours for LiFePO4 chemistry batteries).  That 24 hour “period of total isolation” gives electrons and ions (charged atomic particles) time to reach equilibrium.  The term “natural resting voltage” describes an electrochemical state where the sub-atomic charges in the battery materials have all “found a home” in the atomic structure of their host chemical compounds.  At the atomic level, they are “in equilibrium;” “resting,” not floating about, and not “flowing” anywhere.

    Observe, then, that lithium chemistry batteries in the normal range of charge have an operational terminal voltage that is very close to the nominal “float voltage” (13.3V) of lead-acid batteries.  Once connected together into a bank, absent fault events, a lead-acid sibling rarely has any work to do in the bank.  Compared to the lead-acid sibling, the voltage discharge curve of the LFP sibling is flatter and stays at a higher voltage level throughout most of its usable discharge cycle.  It simply lives there, symbiotically, neither contributing nor burdening.  The lithium chemistry sibling “does the work” of providing DC power to the boat.  The lead-acid sibling will enjoy a relatively long service life because to it, there are rarely any discharge cycles, and its LFP sibling(s) continuously maintains it at its “float” voltage.

    Pro Tip: this lithium/lead-acid hybrid pair can be charged in the same way – and with the same charging equipment and charging voltage profile – as a lead-acid AGM battery.

    Making the Connection

    Connecting lead-acid and lithium chemistry batteries together in parallel can be a non-trivial technical chore which requires some explanation.

    When paralleling two lead-acid batteries, a small spark may occur when the final connection is made.  Fully charged, their terminal voltages and internal charges are nearly identical.  In that case, there will be no significant difference in the electrical charge between them.  Paralleling fully-charged lithium chemistry batteries is similarly non-dramatic.

    When paralleling a LiFePO4 chemistry (13.2V) and a PbSO4 chemistry (12.6V), the difference between the two natural resting voltages isn’t large (600mV), but the amount of electrical charge it takes to “equalize” that difference is actually very significant.  Simply connecting them will result in a dramatic, possibly damaging, spark and a prolonged high current flow from lithium to lead-acid.  To the lithium sibling, the outrush current looks like a low voltage “short circuit.”  The goal here is to get these two dissimilar chemistry batteries paralleled together so that a “surface charge” is built up on the plates of the lead-acid sibling.

    “Surface charge” is a very well understood phenomena with lead-acid batteries.  With fully-charged lead-acid batteries, “surface charge” occurs because there is no longer an available vacancy for free electrons to occupy within the atomic lattice of the lead plates.   When any external charge source is connected to a fully-charged lead-acid battery, a “surface charge” of electrons collects on the surface of the negative plate.  “Surface charge,” alters the “natural” resting equilibrium of the fully-charged battery to a new, and “unstable,” state-of-equilibrium, caused by the presence of the externally impressed voltage applied by the charging source.  In the automotive world, this is exactly what a portable lithium “jump start” pack does to a “dead” lead-acid car battery.  That is, the charging pack delivers electrons to the dead battery faster than the battery can absorb them, and the electrons collect on the surface of the lead plates.

    So, when initially connected together, the lithium chemistry “charging source” instantaneously tries to pump a large current into the lead-acid “load.”  That “outrush current” flowing from the lithium to the lead-acid battery is seen by the BMS as a very large current, and the BMS may trip “off” to protect the lithium battery from damage.  Whence the BMS resets itself, that cycle may repeat.  The result is that the two dissimilar batteries couldn’t be successfully paralleled.

    Enter now, the humble power resistor, such as this one from Amazon.com.
    Think of this resistor as an “installation tool;” a “tool” used at initial setup time.  Like all tools, it’s taken out when needed, used, and put away until it’s needed again.

    The purpose of the resistor is to limit the “equalizing current” so the BMS doesn’t trip. When initially connected, Lithium at 13.2V gives up electrons to its lead-acid sibling at 12.6V. The resistor is placed “in series” in the conductor connecting the two batteries to “limit” that flow of current to a level acceptable to the lithium electron-donor’s BMS.  As arriving electrons build up “surface charge” on the lead-acid plates, the terminal voltage of the lead-acid sibling rises, the voltage difference between the two different chemistry batteries falls, and so the current creating the surface charge in the lead-acid sibling simultaneously falls.

    The surface charge on the lead-acid chemistry battery does not dissipate instantly when the charging source is disconnected.  Since, at initial connection time, there is no electrical system connected to the bank, there is no path into which “surface charge” is able to “bleed.”  So the installer of these connections has enough working time to disconnect and remove the resistor and connect the batteries directly in parallel while that surface charge remains intact.  Since the normal terminal voltages of the lithium sibling (~13.2V) and the lead-acid sibling now with its surface charge established (~13.2V) are equalized at the same (or very nearly the same) voltage, the batteries can be connected directly together without drama or fireworks.

    To initially connect lead-acid batteries into a lithium chemistry bank, the installer will need a large wattage resistor; in the range of 50Ω, 100W.  The bigger the resistance, the lower the current (Ohm’s Law), so the longer the equalization process will take, but the “more controlled” it will be.  Warning: the resistor may get hot to the touch!  The resistor would be needed any time the two dissimilar chemistry batteries need to be connected/reconnected together, so for example:

      1. when first assembled together, or
      2. reconnecting them after a BMS disconnect event, or
      3. when emergency cross-connecting another bank, such as a lead-acid “start” bank, or
      4. after any maintenance activity on the battery bank, like cell/battery replacement.

    For more information on hybrid banks and connecting dissimilar chemistry batteries, review this Youtube video.  The presenter talks through the above entire scenario in detail.  Clark Willix is a boater with many years of liveaboard experience and an engaging speaking style.  He has a 1500 aHr lead-acid bank of L-16s, so way, way bigger than most of us.  But that said, he presents sound conceptual descriptions.

    Pro Tip: whether the above procedure is necessary at all will depend on the relative charge capacities (aHr) of the lithium and lead-acid siblings.  A relatively smaller capacity lithium sibling connecting to a relatively larger capacity lead-acid sibling is more likely to trip the BMS.  A relatively larger capacity lithium sibling connecting to a relatively smaller capacity lead-acid sibling could work without significant drama with a suitably-rated battery paralleling switch.  Arc suppression at the switch will prolong the service life of the mating surfaces of solenoid or switch contactor points.

    Once connected together, there is a permanent, small circulating current (the IFloat current for the lead-acid sibling).  The dissimilar natural voltages are “equalized” between the two dissimilar chemistries, and the lead-acid float current load is very low between them.  That IFloat current is a “Parasitic Load” to the lithium chemistry battery.  Parasitic loads consume power and are oft forgotten.  On boats, “always on” equipment can be viewed as a “parasitic load,” such as digital radio station memories, digital clocks, smoke alarms, intrusion alarms, doorbell cameras, CO/COdetectors, propane gas detectors, bilge pumps, etc.  Parasitic loads can “kill” batteries.  Readers are encouraged to view this “float current parasitic load” as a small price to pay in return for a potentially huge reliability benefit (reward!) provided to the boat’s “Electrical System.”

    Pro Tip: a useful device to install on all boats is a “boat monitor,” such as the Siren Marine “MTC,” or equivalent.  The device notifies boat owners by text message and/or email the moment shore power is lost.  They monitor system B+ voltage, and can notify owners of low house battery voltage.  These devices can be viewed as “parasitic loads,” because they are “always on,” and their presence forgotten, even when batteries are not being charged.  However, these devices can save thousands of dollars in damaged batteries.  A text that reports lost shore power indicates there’s a problem at the boat, and a timely phone call to the boatyard or marina can avoid/save great inconvenience and unnecessary costs.

    Pro Tip: plan ahead for lengthy dockside stays and long-term boat storage lay-up periods.  The lithium industry does not have good solutions for managing “float” charging on shore power.  Lithium chemistry batteries do not like to be left fully charged unless that is part of ongoing discharge and charge cycling, and they are subject to self-discharge when chargers are discontinued.  Owners of boats hauled and left in storage for some months, or otherwise unattended, must make provisions for the care of lithium chemistry batteries.  Lithium solutions in 1Q2021 are for cruisers, not marina mavens.  Plan to leave LFPs between 50% and 70% charged, and NOT on a full-voltage trickle charger.  With proper equipment (adjustable-voltage charger), one can treat lithium chemistry batteries as in an off-grid solar application, cycling them between 50% and 70% SOC.  Maybe with a solar panel and suitably adjusted solar controller…

    Pro Tip: a design requirement for any BMS is that the BMS itself must be disconnected from the battery in the case of an LVC event.  If the BMS stays connected to the battery AFTER an LVC, the BMS itself becomes a parasitic load.  In an unattended situation, like summer layup, that small continuing parasitic load will, itself, lead to permanent damage to (destruction of) the host battery.

    Technical Guidance on Lithium Chemistry Batteries

    ABYC has published a “Technical Report” entitled “Lithium Ion Batteries” (TE-13, July, 2020). That document is not yet an “ABYC standard” to which boat builders, equipment builders, technicians, and surveyors are expected to adhere, but it’s an important guidance document for those interested in lithium chemistry battery solution design.  Some quotes:

    13.5.6 If a shutdown condition is approaching, a BMS should notify the operator with a visual and/or audible alarm prior to disconnecting the battery from the DC system.

    13.9.4 HVE/HVC/LVE/LVC – A BMS should protect the lithium ion battery cells in response to an HVE and an LVE.

    13.9.4.1 Means of protection should not disconnect critical loads without prior warning and should not stop the charging source in a manner that causes damage to the charging device.

    EMPHASIS ADDED in 13.9.4.1 is the authors.  “How” this good guidance is accomplished today remains at the discretion and responsibility of the designer of the  “electrical system,” through specification of equipment in the technical specification of the bill-of-materials.

    My interpretation is, if there is an impending High Voltage Event (High Voltage Cutoff) or Low Voltage Event (Low Voltage Cutoff) – or if there were an impending High Temperature Cutoff – the BMS SHOULD provide a warning in advance of a BMS Disconnect.  The ABYC document does not specify the lead time in advance of the disconnect or the duration of the pre-alarm signal, but this would be something to look for in the manufacturer’s spec sheet before buying a BMS or before buying any of the commercially available “drop-in” batteries.

    There is no specific guidance in the ABYC document on a need for a single “Operation Over-Watch” or “Operation Overlord” BMS intended to protect lithium chemistry systems made up of multiple individual 12V/24V “drop-in” units.  But, here is what the document does say (again, emphasis is mine):

    3.5.1 All lithium ion battery systems should have a BMS installed to prevent damage to the battery and provide for battery shutoff if potentially dangerous conditions exist.
                NOTE: BMS can be external or internal to the battery.

    Clearly, a battery bank composed of multiple “drop-in” LFP batteries does constitute a “lithium ion battery system.”  In 1Q2021, the BMS component(s) needed to accomplish 13.5.1 with a bank of “drop-in” batteries, and the engineering of the “drop-in” batteries themselves, is/are not widely available in the marine market as commercially-manufactured offerings.

    Article Summary

    Lithium-Iron-Phosphate battery alternatives (using prismatic cells) are practical on boats in 2021.  “Drop-in” LFP alternatives exist, but systems to control them are immature and incomplete.

    This article has focused on boats used for long-distance cruising, characterized by frequent, repetitive battery cycling profiles (charge/discharge cycles) and a very high dependency on reliable, always-available “electrical systems.”  The complexity of LFP conversion and operation isn’t technically justified in 2021 for those who’s preferred lifestyle involves traveling from marina to marine or anchors out only occasionally.  The industry is still trying to figure out the best way to handle extended time on shore power; i.e., boats in infrequent, day-use or prolonged periods at dockside on shore power (boats used as seasonal condos).

    Readers that have reached this summary have seen that lithium chemistry battery installations are complex and do have some potentially negative technical impacts.  The net is, as of 1Q2021, there is not yet “expert consensus” on the “optimum design” of a lithium-based SYSTEM; that is, configuration variants that need to be supported and how those variants can, should and need to be controlled and protected for their safety and for the overall SAFETY and RELIABILITY of the host vessel and its crew.  A “dark ship” event is a super big deal, even on a sunny afternoon in uncrowded waters.  It would be a particularly unwelcome event coming into a lock on the Tenn-Tom Waterway or the Waterford Flight on the Erie Canal, or coming into Barnegat Inlet or Ft. Pierce inlet at max ebb (running from a thunderstorm), or crossing the Columbia River bar any time!   We boaters do not yet have the control systems and equipment that will be the “necessary, normal, safe and reliable” platform in just 5 – 10 years time.

    Lithium chemistry systems are, in 1Q2021, an “emerging technology;” NOT YET an “install and forget” platform.  Engineers, manufacturers, boatyards and electrical technicians continue to learn daily from the failure events experienced by “early adopters.”  There are still no accepted industry standards for lithium chemistry “systems,” and the existing UL Standards for lithium battery cells refer to batteries with very small amounts of lithium metal (<5 grams), as found in cell phones and tablet PCs.  There is still way too much “technical stuff” for individual boat owners to have to know.  There are still too few technicians trained to work on lithium systems.  Costs are still very high.  Mistakes are very expensive.  Lithium batteries are subject to permanent damage if “abused.”  Long service lifetime claims – although better than lead-acid batteries – are not borne out in “real life” by “early adopters.”   LFPs should not be installed in engine rooms (heat).  These systems will probably not fail where they were built and installed, so it’s likely the owners, or someone the owner has to locate and hire, will have to deal with any future failure event without benefit of the original builder’s design assumptions, drawings, documentation or assistance; a scenario which leads to “system re-design on the fly…    …at lots of additional cost…”

    A quote from an experienced friend who’s made this transition: “1) Don’t do it just because you can, and 2) if you do, be damn sure you understand how to do it right.”

    The net is, it’s on you as the buyer/owner/operator to ensure your lithium chemistry platform is as reliable from a design point-of-view as it can be before accepting it from the builder.  That means you need to know a lot about lithium technology and “electrical systems.”  It’s also on you to have current electrical diagrams and manufacturer names, part numbers, user manuals and contact information, on-board and available, for all components of the entire system.  If that system needs “repair” at some future time in some exotic cruising locale, you’ll at least have the reference documentation you’ll need to get help.

    For the Serious DIYer

    Serious DIYers interested in comparing lithium batteries to lead-acid batteries should view this analysis.  A very clear lesson that emerges from the data presented here is the cost effectiveness of flooded wet cells compared to AGMs.

    For those who are truly interested in lithium chemistry batteries, and also have SIGNIFICANT DIY ELECTRICAL SKILLS, TECHNICAL CURIOSITY, TOLERANCE FOR POSSIBLE DISRUPTION, A CRUISING BOAT USE PROFILE and at least SEMI-DEEP POCKETS, here is a reasonable 1Q2021 lithium chemistry platform option for you.  In my opinion, this is the most cost-effective way to move to lithium technology in 1Q2021.  I would only undertake this conversion if I planned to own the boat for 10 – 12 more years.  In less time, there is essentially no opportunity for ROI, and the remaining advantages of lithium chemistry over lead-acid are less appealing without ROI as a reward.  Lithium technology has great potential; but that potential is still a few year away for the needs/wants/desires of the “install and forget” buyer/owner.

Earthing and Grounding

11/6/2020: Initial Post
11/16/2020: Text and Graphics added
2/9/2021: Graphics added; minor edits

Introduction

This is an introductory article, written to provide a basic understanding of a complex aspect of AC electric systems to an audience with little or no prior background in electricity. This subject is fundamental to AC system wiring in buildings and on boats, and is a prominent underlying part of the discussion in many other articles about AC Shore Power found on this website.

The concepts around “earthing” and “grounding” are at the very core of making electrical systems as safe as possible to people, pets, farm animals and wildlife. But, “earthing” and “grounding” may or may not mean the same thing when used in conversations and when used without context. These subtle concepts and the terminology they involve can be new and confusing to people without prior electrical backgrounds, and are among the most important to electrical safety. “Grounds” and “grounding” are topics that embrace multiple related ideas. “Earthing” and “Grounding” have different implications in residential single-family house settings than they do on boats, and residential electricians often are not aware of issues that apply to electrical safety on boats. Context is very important to understanding these issues, and as always in electricity, there are many “language shortcuts” that occur in group discussions on docks. Boaters will benefit from an understanding of these topics.

Static Electricity/Lightening

In nature, there is a form of electricity called “static electricity.” A major characteristic of static electricity is that it “flows” outside of wired electrical circuits, through the air. Its flow is intermittent and spontaneous. Static electricity is caused by the friction of two surfaces moving across one another. Static electricity results from the accumulation of electrons on one object (“negative charge”) and a deficit of electrons on another object (“positive charge”). It occurs where friction between surfaces creates a negative charge on the surface with excess electrons and a positive charge on the surface from which electrons were taken.

Residents of low-humidity, cold climates are familiar with static electricity. Little static shocks result from walking across a carpeted room in wool socks, petting the cat or dog, putting on a sweater or overcoat, and then contacting a doorknob, another person, or a car door (or any number of similar life activities).

“St. Elmo’s Fire” is a visible ionized corona; a static electricity “charge” that occurs when conditions are still and humid. In St. Elmo’s Fire, a sphere of blue or purple ionized plasma forms at the sharp points of outdoor structures, such as electric utility towers, church spires, chimney’s, masts, spars, flag poles, weather vanes, etc.  In the far upper atmosphere, a similar phenomena is responsible for the “Northern Lights.”

In clouds, warm, rising water droplets collide with cold, descending ice crystals, causing static charge to accumulate and eventually result in lightening.  The “discharge” of static electricity is a visible flash – an “electric arc” – composed of electrons flowing through ionized air. Electrons “flowing” between two points is the definition of an “electric current.” With static electricity, a voltage difference (electric charge) between the two poles of the static system becomes instantaneously great enough that the insulating characteristic of the normally nonconductive air gap breaks down and conducts. In household situations, the arc is mainly a nuisance, although it can damage modern semiconductor electronics and the “shock,” together with an occasionally audible “snap,” can scare/surprise its animal and human victims.

Lightening is by far the most impressive static electricity discharge phenomena with which we are all familiar. Lightening is static electricity with a massive visible arc composed of many, many thousands of amps. That arc current creates many thousands of degrees of instantaneous temperature rise in the surrounding air, resulting in thunder. Lightening releases massive amounts of energy (mega-joules) and often results in severe damage at its earth contact point. Lightening is more than capable of killing animals and people.

The arc of a static discharge “neutralizes” the accumulated positive and negative atomic charge of the oppositely charged poles of the static “system.” Protecting building electrical systems from being damaged by the discharge arc of lightening involves creating a means to get the arc current to flow AROUND, rather than THROUGH, the electrical system of the building, or its structural components. To protect a building, metallic “air terminals” are placed high, on roofs. A network of heavy electrical conductors connect the air terminals to rods driven into the earth. Large communications towers, bridges and high rise buildings often utilize their own metallic structure as a safe path for guiding discharge currents into the earth. Farm structures (barns, grain elevators, windmill pumps, etc), industrial sites (refineries, chemical plants, chimneys, etc, etc), and hospitals are protected with air terminals and metallic paths to earth ground. These protective devices are apparently considered unsightly and undesirable in suburbia, because they are rarely found on single-family residential buildings. When we lived in Indiana, our neighbor across the street had the chimney blown off his house by a lightening strike to that unprotected structure.

Lightening protection for boats is a separate and complex study; inexact, expensive to install, and impossible to properly retrofit if not built into the initial design at the construction phase of the boat’s life. Boats struck by lightening almost always experience severe electrical system damage and extensive damage to electronic equipment aboard. Lightening can literally blow a hole in a boat’s hull on its way to earth ground.

See my article on “Faraday Cages” for ways to protect sensitive electronic gadgets from lightening; for example, hand-held VHF radios, hand-held GPS, computers, back-up hard drives and cellular telephones.

Residential Electric Circuit “Wire” Naming and Identification

All operational electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”

In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and can also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the DC color code was changed to “yellow” to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (almost always) not technically correct, since “ground” wires are not intended to carry current in normally operating systems (reasons explained later).

In AC circuits in buildings, the power on the conductors is alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work. In single phase 120V circuits in North America, the two conductors are named for their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained later), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

In electrical engineering, “earth” is the single reference point in an electrical system from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the National Electric Code for AC distribution circuits in buildings has required “Equipment Bonding Conductors” and an “Equipment Grounding Conductor.” In the NEC, Article 250 is the standard for “grounding and bonding.” Each individual conductor that is an individual component that comprises the network of conductors that make up the “ground system” has its own specific name. For the purpose of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”

The NEC, Article 100, defines an “Effective Ground-Fault Path” as an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origination point of a ground-fault in a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault sensors. The purpose of the safety ground is to create an “effective ground-fault path.” That low resistance path is intended to function as a fault-clearing path; for that single “emergency use” only. “Fault-clearing” means that the circuit breaker feeding that faulting circuit will trip to remove power from the circuit. Under normal conditions in a properly wired electrical system, the safety ground conductors (including the bonding system network on a boat) DO NOT/MUST NOT carry current in normal, routine operation. The safety ground conductors are intended to ONLY carry current when there is a “fault” in the system. In buildings on land, the ground conductor is typically bare copper wire. On boats and in appliances, the ground wire is insulated, and solid green in color, or green with a yellow stripe.

Subtle take-away: the DC “negative” conductor has the same role in a DC circuit that the AC “neutral” conductor has in a residential/boat AC circuit. That is, the DC “Negative” conductor returns current from the load to the power source (battery). In a “grounded DC electrical system” (which is uncommon), there is a third conductor that is part of the DC circuit, just as there is a safety ground in a 120V AC system. The B- conductor is a “Current-Carrying, Grounded Conductor,” and is entirely separate from the actual ground conductor.

NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new and repair work, always install the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape. The NEC does not allow green safety ground wiring to be changed. Safety ground wiring must be green, and green must not be used for any other purpose.

Note: on some but not all boats built overseas, AC wire colors may be different than the North American standard (NEC and ABYC) colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white. This difference is also common on boats built overseas, because they follow the European color standards. If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine how that wiring is used to ensure equipment, fire and personnel safety.

However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.

Electrical Circuits

Core concept: opposite to the situation with static electricity, in man-made electrical circuits, the electricity originates at a point that is know to be its “source.” This can be a battery, a solar cell, a fuel cell, a generator, or a point-of-connection to the electrical grid. An electric “circuit” is said to exist when an electric “current” has a path that enables electrons to flow out of the source on a conductor, travel through a load to do useful work, and then return to its source on another conductor. The “source” can be DC or AC. Whether DC or AC, a “voltage” can appear at the output terminals of a source (like a battery or generator), but a “circuit” does not exist unless electrons can flow out of the source, through a load, and back into the source. A “circuit” consists of is a round trip of continuous conductive wiring for current flow out of a source and back into the source. A “switch” is any electrical device that “opens” a circuit to prevent electron flow as a matter of convenience and/or function; a relay is a device that interrupts current flow in the circuit that it controls; a “fuse” or “circuit breaker” is a device that “opens” a circuit to protect conductor insulation or remove power as a matter of fire prevention and/or personnel safety; and, a “severed” (“broken”) wire is a “malfunction” (“fault”) that “opens” a circuit so that there is, in effect, no round-trip circuit for electrons.

Fundamental Physics of Electric Circuits

Rule 1: Electric currents MUST RETURN TO THEIR OWN SOURCE.
Rule 2: Electric current will return to its source on ALL AVAILABLE PATHS. Corollary: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path.
Rule 3: An electric “circuit” does not exist UNLESS current has a continuous conductive path on which to flow from source back to source.

Readers will come back to these fundamental rules of electrical behavior over-and-over again when dealing with electrical systems and the concept of electrical faults. The more complex the electrical system, the more numerous and complex the issues, but electrical safety always comes back to the physics that underlies the behavior of electric currents.

The National Electric Code (NEC) and the American Boat and Yacht Council (ABYC) electrical standard, E-11, provide design and installation requirements that define system controls that manage how voltages will be safely removed and currents will be safely stopped (disconnected) in response to faults of various kinds that may occur in an electrical system. It is actually quite easy “to get something to work.” It is much more complicated and much more important to control electricity when something isn’t right. Disconnecting power, and disconnecting power safely, is the only way to prevent fires and electric shock risks to personnel.

Electric Code Grounding Categories

Finally, we get to “earthing” and “grounding.” There are two contexts for electrical “grounding” as required by the NEC.

  1. System Grounding
  2. Equipment Grounding (Bonding)

Residential System Grounding

“Ground” is the standard reference point for measurement of voltages. The NEC, Article 100, defines the crust of our beloved home planet as “Ground.” Ergo, Sir Knight, the electrical potential (natural voltage) of the earth’s “soil” is defined to be “zero volts.” All voltages are measured from an earth ground reference point.

The crust of the earth is electrically conductive. The earth’s crust contains many minerals and mineral salts which provide “free electrons.” In response to an impressed voltage, electrons will flow from point-to-point around and within the earth’s crust. An important corollary is that currents flowing in the crust of the earth follow the fundamental rules of electro-physics, including “Ohm’s Law” and “Kirchhoff’s Law.” In order to create a residential electrical system connection to “earth ground,” one or more interconnected metallic rods (often copper) are driven into the earth.

In the North American residential AC system model, three conductors arise from the utility power transformer at the street. All three are “Current Carrying Conductors.” Two of those conductors are considered, by conventional agreement, to be “energized” (“L1” and “L2”) and one is the neutral line (“N”). This is known as a “Single Phase, Center Tapped, Three-Pole,” system. The “Neutral is the transformer’s center-tap connection. As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V is present between “L1” and “N” and between “L2” and “N.” Note, however, that at the street, these voltages “float” with respect to their external environmental surroundings. They are not connected to anything. This situation is referred to as a “floating neutral system,” and in a “floating neutral system,” the voltage between the neutral and earth ground is unlikely to be “zero.”

If these three lines were connected to a distribution panel in a residence, all electrical appliances would work correctly. All of the necessary operating voltages inside the building would be correct. But, measured against a ground reference, it’s entirely likely the neutral would be at some perhaps large voltage difference with respect to the metal sink where food is prepared, or the metal bathtub when the baby gets bathed, or the metal faucets in the family shower. Clearly, a shock hazard would exist. To eliminate that hazard, the “Neutral” is electrically “tied” (connected) to an earth-ground reference point.

To create a system referenced to a known. zero-volt earth ground, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. This connection results in an earth-ground, “grounded neutral” system, throughout the premises. In a grounded neutral system, the voltage between the neutral conductor and the safety ground conductor is “zero,” or should be very close to “zero.”

While it’s true that the earth is electrically conductive, the earth is not a good conductor. Even at its best, “dirt” is not as good at conducting electricity as aluminum and copper wire (and also not as good as salt water). But rest assured, Ohm’s Law is a fixed “law” of physics, and it does apply to currents flowing in the earth. So while “dirt” may not be a great conductor, it is a very large-diameter conductor, with an infinite number of parallel paths, and with virtually unlimited ampacity. Just how well any local parcel of “dirt” conducts electricity depends on many things, including mineral and moisture content. The NEC requires that ground rods have a minimum contact resistance of 25Ω to earth. Sometimes, that can be achieved with a single 10′ rod driven into the soil; sometimes it requires a long rod driven 40′ – 50′ into the ground; and, sometimes it requires an entire network of long ground rods, all driven deep, and all connected together in parallel.

The essential point here is that “earth ground” is a universal reference point for all terrestrial power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the absence of any voltage. This works well because in a properly functioning, properly wired system, no current flows on the grounding system. Since no current flows, the voltage at the contact point with the copper grounding rods stays reliably at zero volts (as predicted by Ohm’s Law). Electrical faults (discussed later) create vastly different, sometime dangerous conditions.

Important to realize in this discussion, the earth ground alone DOES NOT protect against electric shock. It is merely a reference point against which system voltages are stabilized at “zero.” Earth ground IS NOT a reference for protective devices (fuses, circuit breakers) to trip to remove power when an electrical fault condition occurs. The Earth IS NOT the “source” for any DC or AC electrical energy. Remember Rule 1: “All electric currents MUST RETURN TO THEIR OWN SOURCE.” Electrical currents in residential and boat electrical systems DO NOT originate in the earth, and so, do not return to the earth. However, under some kinds of fault conditions, current can and does return to its source by traveling through the “dirt;” or, through the water in which a boat is floating!

Well then, why do we have the “Earthing” connection? Well, “Earthing/Grounding” in this context is a single-point-of-connection (one point and ONLY ONE POINT) to the earth for the purposes of mitigating:

  1. Static build-up (wind induced),
  2. System voltage instability, including:
    ▪ Unintentional physical contact with a higher voltage system (automobile accident or severe weather incident involving “hot” utility services),
    ▪ Repetitive intermittent short circuits (dispatched to first responders as “trees on wires, burning!”), and
    ▪ Utility switchyard and distribution system switching surges (spikes).
  3. Nearby vicinity lightning splash, and
  4. Transient interference (from static discharge and local RF emissions).

Not all of these exceptional conditions apply equally to all residential premises systems, but because some do apply in all areas, the National Electric Code treats all alike.

Faults

Consider a building’s main electrical service panel as the “source” of AC power (volts and amps) for the building and all of its branch circuits. In a household AC electrical system, current from that source emerges from a wall outlet on one appliance conductor and returns to the wall outlet on the other appliance conductor. Refer to Rule 1: “Electric currents MUST RETURN TO THEIR OWN SOURCE.”

Now consider a hot water heater, washing machine, trash compactor, dish washer, garbage disposal, microwave or toaster oven, each constructed with a metal exterior cabinet. The appliance is an electrical “load.” Electricity is provided to it from the wall and returns from it to the wall. With just two conductors (supply and return), the appliance can work normally. But, what happens if there is a frayed or cut wire inside the cabinet, and in physical contact with the metal cabinet of the appliance? In that event, the cabinet will have a non-zero “touch potential” (voltage) on it’s metal enclosure, and that voltage could easily be a shock hazard to residents. This is exactly how houses were wired before the 1950s, and many people reading this will remember the “two prong” duplex outlets of that time. In those days, people did get shocks from household appliances, fans and table lamps. Sometimes even from the iron. (Did Granny’s iron have fraying cotton insulation at the plug end? Does anyone actually iron anymore?) And sometimes, the shocks were serious. These shocks were the results of “faults” in the circuit.

A “fault” is said to exist when:
1) an electric current does not flow when it should, or
2) an electric current flows in an unintended path to get back to its source.

Clearly, an electric shock – which is a path through a person’s body – is an unintended path. To avoid shocking experiences like this, a third electrical conductor (safety ground) was added to electrical systems in homes, garages, barns, workshops, supermarkets, retail stores, office buildings, malls, commercial offices, workplaces, etc. That is, anywhere people might come into contact with electricity.

Grounding Conductor

This brings us to the next major category of “grounds” and “grounding.” Not to the earth itself, although it is connected to the earth, but rather to a common point in the building’s main electric panel. In this context, the word “ground” is a useful – but misleading – concept, because the ground conductor does not live in the ground and it does not send fault current into the ground. The ground conductor is connected to the ground rods at the service entrance, so it is REFERENCED to ground. That way, that ground conductor is held at “zero” volts with respect to all other components in the electrical system.

The “equipment grounding conductor” (a/k/a the “safety ground”) in residential and boat electrical systems is designed and intended to cause fuses or circuit breakers to trip in order to DISCONNECT POWER in case of a fault. It is, in the words of the NEC, an “Effective Ground-Fault Current Path;” that is, an intentionally constructed, low-resistance, conductive path designed to carry fault current from the origin point of a ground fault in a wiring system to the electrical supply’s source and that facilitates the operation of the overcurrent protective device or ground-fault sensors.

Disconnecting power is the ONLY WAY to protect against fire and personal injury caused by ground faults in an electrical system. Equipment grounding is the intentional (in fact, NEC Article 250.2 mandatory) act of providing a network of conductors that interconnects the metallic cases of all electrical equipment attached to an electrical distribution panel. The bare copper or green-insulated “grounding conductor” discussed earlier is connected to the metallic cabinets of all modern appliances, and to the round ground pin of North American 15A and 20A household electrical utility outlets. The wires that make up the network of grounding conductors in a home have several names, but “safety ground” is representative for this discussion. On a boat, this network of green grounds is called the “bonding system,” of which the AC Safety Ground is a key part.

Residential dwelling units in North America range from tiny houses to single family homes to compounds with outbuildings to multi-family buildings of all kinds. A “ground buss” is always located in the main service panel of a dwelling unit, and in any sub-panels that may be supplied from that main service panel. Ground conductors from all branch circuits in the panel are connected together at the panel’s “ground buss.” Sub-panel grounds are in turn brought back to the ground buss in the main service panel. Boats are wired as sub-panels, not as main service panels.

At ONE PLACE in the main electrical service panel of the building, the “Grounding Conductor” is electrically connected (bonded) to the “Neutral” “Current Carrying Conductor.” By code, there is ONLY ONE “Neutral-to-Ground” bond in a residential electrical system, and it is placed at the Main Service Panel – never in sub-panels. A boat is wired as a sub-panel, so there should NEVER be a neutral-to-ground bond aboard a boat connected to, and operating on, shore power. This mistake in wiring on a boat is a very common cause of boats tripping shore power ground fault sensors on docks.

Now consider the fault case where an internal fault of some amount tries to put a touch potential voltage on the metal cabinet of an appliance. Rule 2 applies; “electricity will return to the source on all available paths.” Since the grounding conductor is attached to that metal cabinet, the Grounding Conductor does two things. First, it holds the voltage of the appliance cabinet at zero volts (because it’s “grounded” at the main service panel to the network of ground rods), which protects people and pets from shock. Second, it provides a very low-resistance path back to the service panel, via the neutral-to-ground connection, which instantaneously draws a very large spike of current through the circuit breaker (or fuse). That instantaneous large overload trips the circuit breaker to REMOVE POWER from the faulting circuit. Removing power is how the system protects buildings against fire and protects people from electric shock.

Ground Faults

The earth’s crust is electrically conductive, so that creates two electrical system design and code issues.

Rule 1 again: Electric currents MUST RETURN TO THEIR OWN SOURCE; and
Rule 2 again: Electric current will return to its source on ALL AVAILABLE PATHS;

Enter, our Corollary to Rule 2:: if there are parallel paths back to the source, current will divide and some portion of the total will take each available path. This law of physics is called Kirchhoff’s Law, which states that when there are multiple parallel paths back to the source, current will divide and some portion of the total will take each available path back to its source.

In both home appliances and boat appliances, the two most common causes of “ground faults” are aging water heater elements and aging motor/transformer windings. In a water heater, power can leak through the water in the heater between the energized heating element and the metallic case of the water heater. In a motor, over time, dust and other airborne contaminants build up in motor windings, and at the same time, heating and cooling cycles cause the winding’s insulation to break down and develop micro-pores. In these cases, the fault current isn’t enough to trip a circuit breaker, but small amounts of power can leak to the Grounding Conductor, and then back to their source at the main service entrance panel. This is a ground fault by definition, because ANY current flowing on the safety ground is flowing on an unintended path. In this case, the fault current flows back to the source on the Safety ground’s conductor. More in a couple of paragraphs, but first, some illustrations.

Here’s a homeowner scenario… Dad’s gonna trim up the lawn, trim some plants, and wash the car (he’s young and energetic, unlike myself). He runs a 100′ extension cord in order to power an electric hedge trimmer, grass trimmer, circular saw, reciprocating saw, radio, charcoal fire starter, polisher/buffer, whatever. The extension cord has a ground wire, but the “tools” attached to it by multi-outlet adapter either have only two wires, or the ground pin has been cut off as a “matter of portability convenience.” Tools that aren’t actively in use are lying on the ground, where they and their cords are in contact with the ground. Now there is a path for power to get back to its source through the soil, to the ground rod(s) serving the main electric panel, and back to the neutral in the main service panel. That is a ” ground fault” because it is clearly an unintended and unwanted electrical path through the soil (ground). And at some point in this scenario, Dad will pick up his tools and possibly have a shocking experience. Possibly even, a lethal shocking experience. Without a continuous “effective fault-clearing path,” there is no way to shut off the power to save Dad from a shocking experience

OK, here’s another scenario with which my daughter and I have direct, personal experience. One Halloween “Hell Night,” Kate came home in need of a shower to remove 17 cans of different brands of shaving creme and lord-only knows what else she had encountered while “out with friends.” She went off to the shower, whereupon Peg and I laughed at her state of dishevelment! Note here, one of our sons had just finished his shower from his night “out with friends.” After just a couple of minutes, there arouse a righteous and shrill scream from the upper reaches:

“Daddy! Turn the water back on!”

In my total, complete and absolute innocence, I grunted at Peg: “Huh?”

The house water pressure had disappeared to a dribble while Kate was all lathered up. Mid-shower! Springing into action, Mom was “off to the rescue,” and Dad was “off to the basement.” In the basement, all seemed OK, but alas, there was no house water pressure.

Plumbing leaks? No water on the floor!
Pressure in the well tank? No! Gauge reading “zero.”
Pump Circuit Breaker “on?” Yes; and not tripped.
Pump relay OK? Yes, relay “picked.”

“Uh oh!” “Darn it!” (or words to that effect)! “Must be the well pump!”

Our homestead in the Catskill Mountains – and all of our neighbors – had a private deep-well that supplied our drinking water.  Our well was 100′ deep, and the pump lived at the 90′ level (not very deep). As the pump started and stopped over many years, it twisted (torquing) on the end of 90′ of semi-flexible PVC hose. The wires running to the pump abraded against the earth and rock walls of the well, and eventually the wire’s insulation wore through. This created a ground fault connection from the exposed bare wire directly to the earth about 70′ down.

Deep well pumps are usually two-wire, 240V circuits. One conductor of ours was in direct contact with the wall of the well. If the point-of-contact had been within the cast iron portion of well casing, it’s likely the circuit breaker would have tripped, because that metal casing did have an equipment grounding conductor. But in our case the point-of-contact was with sediment or rock, the 240V circuit breaker indeed did not trip. That did, however, create a significant ground fault. The pump was trying to start, but didn’t get enough voltage to overcome the weight of a 90′ column of water. Power was flowing into the earth, but not enough to overload and trip the pump’s circuit breaker. Power divided where the bare wire touched the well’s wall. Some of the power going down that hole got to the pump and returned on the other current carrying conductor, but some of the power going down that hole flowed back to the panel through the earth, to our home’s ground rods, and back to the service panel’s neutral.

In these situations, a newly-installed (since 2002 or so) residential service panel would have been fit with “Ground Fault Circuit Interrupter” (GFCI) to remove power and terminate the ground fault condition. In the case of yard tools creating a shock hazard at the end of an extension cord, GFCI could literally save Dad’s life. In the case of the deep well fault, GFCI could have saved equipment from damage. Our deep-well pump got burned out by the prolonged stall created by the low supply voltage. Relate this to boats on docks with pedestals fit with 30mA “Equipment Protective Devices.” This is a case where a 30mA EPD on the well supply would have saved the well pump from damage, and would have provided a clear hint to the location and nature of the fault.

GFCIs and EPDs work by monitoring the outgoing and returning current on the two Current Carrying Conductors. The currents should balance equally between the two conductors. If not, there is a ground fault and the GFCI device trips power off. What happens if there is no GFCI, as was our case at that time? Well then, the ground fault condition continues, because power flows out from the source, but has multiple parallel return paths, one through the returning current carrying conductor and the other through the earth to the ground rods at the main service panel at the same time.

See my article on causes of ground faults on boats for information specific to that topic.

See my article on GFCIs for more detail on how these devices work.

Ground faults on boats behave in the same manner, but are very dangerous, because instead of flowing through dirt, which is largely inaccessible to people, pets and wildlife, ground faults on boats can and do flow through the water. People – especially children – pets and wildlife are sometimes found in the water.

See my article on “Electric Shock Drowning” to read about ground faults in the water.

Ground faults on land can be quite dangerous in another, subtly different way. Suppose a 240V mercury arc exterior driveway light has a ground fault at the pole base that is not large enough to trip an over-current circuit breaker. We all now know from my well scenario, above, that 240V in direct contact with the earth will probably not trip a circuit breaker. But in that condition, the soil surrounding the point-of-contact between the energized conductor and the soil itself is electrically “hot.” This condition sets up a “voltage gradient” on the surface soil surrounding the point-of-contact. Using 240V in this example, at the point-of-contact with the voltage, the voltage in the soil is the same as the supply voltage, so there is no DIFFERENCE in the pole voltage and the soil voltage. But Ohm’s Law applies here, and however much current is flowing into the ground and back to the service entrance panel is creating a voltage drop along the surface of the soil (or driveway). So, the resistance of the local soil matters. One electrical standard1 assumes that 25% of the total voltage drop due to path resistance will be found in the first foot of distance away from the point-of-contact. One foot away from the point-of-contact, the soil is at 163V of shock “step potential.” Three feet from the point-of-contact, the soil is at 202V. Five feet from the point-of-contact, the soil is at 206V. As you can see, straddling the voltage gradient of the surface soil can create dangerous “step potentials” in the soil. Imagine the potential for what could happen when Rover comes over to “mark his spot” at that light pole.

The same sort of voltage gradient forms in the water around the prop and rudder or a boat if there is an AC ground fault on the boat. That gradient is quite enough to get a diver’s undivided attention. If the fault itself is in a heat pump, and the diver is working on the boat when the heat pump cycles “on,” … Well, that diver would quickly know how Rover felt…

See my article on “Electric Shock Drowning” to read about ground fault voltage gradients in the water.

Ground faults can be very dangerous!

Do not defeat safety devices.

Install GFCI and ELCI on boats.

___________________________________

  1. ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) [4.1.1]

Inverters On Boats

7/20/2020: Initial Post

The ABYC definition of an inverter is “an electronic device, powered by batteries, designed primarily to provide AC current at a required voltage and frequency.”  In North America, inverters produce 120V AC (or 240V AC) at 60 Hz from energy stored in 12V or 24V batteries.  On boating forums that I follow, there have recently been many questions about selecting and installing inverters on boats, so in this article, the topic is “Inverters on Boats.”

There are two types of inverter installations found on boats.  The first case is the stand-alone inverter.  These are usually smaller inverters used for charging cell phone batteries or powering portable computers.  Larger stand-alone inverters can be installed alongside, but separate and isolated from, the built-in AC system of the host boat.  Stand-alone inverters are  limited in features, requiring manual intervention each time they are needed.  They are turned “on” manually when needed and turned “off” manually when no longer needed.  Their un-shared outlets are often mounted on the unit itself.

The second case is inverters installed within the host AC power system of a boat.  When installed fully-integrated within a boat’s AC power system, inverters offer boat owners a whole-boat “Uninterruptible Power Supply” (UPS), and commonly function as battery chargers while external AC power is available.  Inverters installed within the host electrical system must comply with cUL/UL-458 per the ABYC Electrical Standards E-11 and A-31.

In 2020, most inverters sold for installation on boats are Pure Sine Wave (PSW) devices.  Older inverters were Modified Sine Wave (MSW) devices.  Some 120V household devices did not work well, sometimes not at all, on MSW inverters.  Generally, PSW devices are to be preferred for overall compatibility with consumer electronics in household equipment and appliances.

Figure 1 shows a stand alone inverter.  Inverters in operation can demand a great deal of DC current from batteries. Regardless of stand-alone or fully integrated installation, the B+ and B- cables from the batteries to the inverter must be sized for the maximum current the inverter can draw from the battery.  The B+ feed must be fused to protect the cables, and should have a disconnect switch rated for continuous use at or exceeding the maximum demand of the inverter.  The device itself must be “grounded” to the grounding buss of the host boat.  Unfortunately, I too often see stand-alone inverters that do not meet these ABYC electrical standard requirements, which apply to all DC devices.

The ABYC electrical standard, E-11, “AC And DC Electrical Systems On Boats,” July, 2018, treats stand-alone inverters in the same way it treats any other DC device (windlass, winch, thruster, water pump, instruments, auto-pilot).  The AC output of a stand-alone inverter is entirely separate and isolated from the boat’s host AC power system.  Thus, there are no specific ABYC requirements for the AC output of a stand-alone inverter.  These devices are easy to install, relatively inexpensive, and can meet basic AC power needs.  Some stand-alone inverters do not comply with North American residential electrical system requirements (grounded-neutral).  Stand-alone inverters enable bad user practices, such as extension cords running across the floor of a boat, and wiring that is too small for the loads.  A common “operator error” is to forget to turn the stand-alone inverter “off” after use, which can damage or destroy batteries.  These “owner errors” are common as fire and personal safety concerns.

Figure 2 is a “simplified view” of a typical 120V AC shore power system as found on many cruising boats.  I have taken a shortcut to also show that this boat has a generator installed.

The ABYC E-11 electrical standard does apply to this AC system.  In a previous article, I discussed the E-11 Standard as it correlates to Sanctuary’s AC system.

There is an important US National Electric Code/Canadian Standards Association “rule” to remember about all end-user AC power systems in North America.  For fire and shock safety, AC power sources are grounded at their source.  The result is called a “grounded-neutral” system.  The neutral conductor itself is a current-carrying conductor that returns current from the load to its source.  To automatically disconnect electrical faults, the neutral conductor is held at zero volts by a connection between the neutral conductor and the facility’s ground conductor.  The connection is called the “neutral-to-ground bond,” or “System Bonding Jumper.”  So in Figure 2, the shore power neutral conductor is “bonded to” the shore power ground conductor before these conductors come onto the boat, in the electrical infrastructure of the marina/boatyard.  The neutral of the boat’s onboard generator is “bonded to” the boat’s AC safety ground network at the metal frame of the generator.

The “grounded-neutral” requirement is the reason the “energized” (“hot”) Line conductor AND the “grounded” Neutral conductor must BOTH be switched by the Generator Transfer Switch (GTS).  When the GTS is in the “Shore” position, the neutral-to-ground bond comes onto the boat from the shore facility, via the shore power cord.  When the GTS is in the “Generator” position, the neutral-to-ground bond is at the generator, as shown in Figure 2.  To eliminate a ground fault path, the generator’s neutral-to-ground bond CANNOT also be in the active circuit when shore power is feeding the boat.  So, it is switched “out” of the active circuit by the GTS, which switches both the hot and neutral conductors.

Figure 3 shows the case of an inverter that is fully-integrated into the host AC system of the boat.  In this case, the inverter is not stand-alone, as in Figure 1, but is installed within the host AC system, between any other AC power source(s) and the boat’s AC distribution panel.  Here, it can be operated manually, or it can operate automatically, changing modes as incoming AC power comes and goes.  Automatic operation is helpful when commercial power fails, or when a dock neighbor inadvertently turns “off” the pedestal breaker of another boat.

As shown in Figure 3, power from either shore or the onboard generator is supplied to the inverter’s AC input.  This cUL/UL-458 compliant design operates in one of two modes.

STANDBY mode – passes power that originates upstream of the inverter through to attached downstream loads (“passthru”); in Figure 3, all of the boat’s AC loads are fed via the inverter.

INVERT mode – draws energy from the onboard batteries in order to create AC output at the rated voltage (120V, 240V) and frequency (60Hz) to feed downstream loads.

Figure 4 shows a similar system, but here some loads are powered via the inverter and other loads are powered only by upstream AC sources.  On Sanctuary, our onboard utility outlets are powered via our inverter, but our hot water heater, genset battery charger and fridge only receive AC power from upstream sources.  That arrangement greatly conserves our available battery capacity.

Note that Figures 3 and 4 refer to the Underwriter’s Laboratory’s UL-458 Standard, which is entitled, “Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts.”  Recall that all AC power sources in North America must be grounded at the source (grounded-neutral), and so shore power is grounded in the facility infrastructure and the generator is grounded at the generator.  To accomplish automatic ground switching, inverters intended for use on mobile platforms (ambulances, trucks, airplanes, RVs and boats) MUST comply with cUL/UL-458.

This is a good time to digress for a moment to look at the ABYC portfolio of electrical safety standards.  These standards fall broadly into two categories.  The first is standards that apply to the design and construction of individual electrical components, such as:

    • A-16 Electric Navigation Lights
    • A-27 Alternating Current (AC) Generator Sets
    • A-28 Galvanic Isolators
    • A-31 Battery Chargers And Inverters
    • A-32 AC Power Conversion Equipment And Systems
    • E-10 Storage Batteries

The second is standards which apply to joining individual component parts together to work within a unified boat system, such as:

    • E-11 AC And DC Electrical Systems On Boats
    • E-30 Electric Propulsion Systems
    • H-22 Electric Bilge Pump Systems
    • TE-4 Lightening Protection
    • TE-12 Three Phase Electrical Systems On Boats

All of these standards make reference to other Industry Standard sources for detailed specification of performance requirements.  Typical outside references are to established by  industry standards organizations including IEEE, IEC, ISO, cUL/UL and eTL.

So as applies to inverters, there is an ABYC standard (A-31) that is specific to the design of the unit itself, and a second ABYC standard (E-11) governing the system into which the unit is installed.  For inverters, the design reference is UL-458 in the US (and CSA C22.2#107.1 in Canada).

When a UL-458 compliant inverter is in “Invert” mode, a relay inside the inverter automatically creates the inverter’s neutral-to-ground bond.  When the inverter is in “Standby” mode, that same relay automatically removes the inverter’s internal neutral-to-ground bond so both AC power and the source’s neutral-to-ground bond are “passed through” the inverter to the boat’s AC power panel.  Functionally, this is what a GTS does in the case of a generator; i.e., when the GTS is set to “Shore Power,” the neutral-to-ground bond at the generator is switched out of the system.  The GTS transfers both hot and neutral, and transferring the origin of the neutral is what changes the origin location of the Neutral-to-Ground bond.

Figure 5 shows a simplified drawing of a UL-458 inverter in “Standby” mode.  AC power passes through (“passthru”) the inverter from an external AC power source, whether that be shore power or generator.  The relay shown in the red circle is “energized” (“picked”) by the presence of external AC power, so it connects the incoming power hot and neutral conductors to the output load circuits.  The green circle shows the inverter’s ground connection, but since external power is present, the relay is “picked,” so the neutral-to-ground bond that is located at the incoming source is “passed through” the inverter to protect downstream branch circuits.

Figure 6 shows the same inverter operating in INVERT mode.  In this case, incoming AC power is absent, so the inverter’s internal relay (red circle) is de-energized (“down”).  Because the relay is “down,” AC output from the inverter is created by the inverter’s electronics from energy stored in the boat’s battery bank.

The green circle highlights the inverter’s internal neutral-to-ground bond, which in this mode is connected via the relay.  That connection is required because the inverter, in INVERT mode, is the actual “source” of the AC power being delivered to the boat.

Following in Figure 7 is a complete circuit diagram of the AC system aboard Sanctuary.  Our 120V, 30A, two inlet AC System is fairly common on boats of our size class, and consists of eight AC branch circuits serving the equipment on the boat.  Other than completeness, our system is just like the simplified view portrayed in Figure 4.  Boats with 240V, 50A shore power service (3-pole, 4-wire cords) will look slightly different on the front end, but 120V inverter installations will be the same as shown here.

Sanctuarys generator is in the upper-right corner of the drawing.  Note the generator’s neutral-to-ground bond, highlighted there in green.

Our fully-automatic, fully-integrated inverter/charger is in the lower left-center of the drawing, in the small red circle.

On the right middle, in the dotted red circle, is our house, “Shore 1,” AC distribution panel, containing the eight branch circuits.  The top four branch circuits are fed only from either shore or generator power, whichever is selected by the GTS.  The bottom four branch circuits are fed via “Invert” or “Standby (passthru)” power via the inverter.  Our inverter is always part of our outlet distribution circuit, 24x7x365-1/4.

In Sanctuary’s system, at inverter installation-time, the hot buss feeding the branch circuit breakers on the AC power panel had to be divided into two parts (blue ellipses) in order to accept two separate feeds from 1) external power and 2) the inverter.  Dividing the hot buss required modification of the OEM electrical panel.  Also at installation-time, the neutral buss (red ellipses) had to be divided in order to separate the neutrals of circuits that are not fed via the inverter from the neutrals of circuits that are fed via the inverter.

The need to separate the neutrals stems from the requirements of the 2011 NEC and 2012 ABYC E-11 standard, adopted in coordination to reduce/eliminate dangerous ground fault currents flowing into the water from docks and boats.  (See the article on Electric Shock Drowning for more information.)  If the neutrals are not separated, an unintended ground fault leakage path can be present.  The day the boat arrives at a marina or boatyard where pedestals are fit with ground fault sensing shore power breakers is the day that boat may trip the shore power breaker, and will not be able to get shore power.  The dock attendant will tell the unhappy boat owner that “there is an electrical problem on your boat.”  The unhappy boat owner will think, “but it’s been working for many years!”  Both statements are correct.  It had worked for many years, but there is “an electrical problem on the boat!”

A fundamental rule of all electricity is, current will flow on all available paths to get back to it’s source.  If the neutrals from one AC circuit on the boat are cross-connected to the neutrals of another AC circuit on the boat, power will divide at the cross-connection (neutral buss) and flow back to the source via all available paths.  That situation is, by definition, a ground fault.

Following are two relevant and important excerpts from ABYC E-11, July, 2018:

11.5.3.6 Isolation of Sources – Individual circuits shall not be capable of being energized by more than one source of electrical power at a time.  Each shore power inlet, generator, or inverter is considered a separate source of power.

11.5.3.6.1 Transfer of Power – The transfer of power to a circuit from one source to another shall be made by a means that opens all current-carrying conductors, including neutrals, before closing the alternate source circuit, to maintain isolation of power sources.

Ordinarily we think of cross-connected neutrals as a situation that affects boats fit with two 120V shore power inlets; indeed, the neutrals from those two inlet circuits must not be cross-connected on the boat.  But more subtly, the separation requirement also applies to distribution circuits fed from generators and inverters.  UL-458 is the design standard that specifies that the needed neutral-to-ground bond in an inverter be “established” and “removed” based on operating mode.  If the inverter neutrals and non-inverter neutrals are cross-connected (as, for example, all sharing a common neutral buss on the boat), the terms of 11.5.3.6.1 may not be met, resulting in a short ground fault condition.  In that case, there can be a duplicate path, if only momentarily, for shore power to use to return to the pedestal.  The following events happen in a fraction of a second.  Just “milliseconds (mS).”

At the instant (time=0.000) shore power is applied to the boat, any AC current that comes onto the boat via the hot conductor should also return to the pedestal on the shore power neutral conductor, and ONLY the neutral conductor.  Period!  Full stop!  Fundamental rule!

But…   At the instant shore power is applied to the boat (t=0.000), the inverter is in “Invert” mode with its internal neutral-to-ground bond still in place.  For the time it takes the inverter to respond to shore power and transfer its internal relay from “Invert” mode to “Standby” mode, there are two paths for the newly applied shore power to take to get back to its source at the pedestal.  The first path is via the shore power neutral, as intended.  But with unseparated neutrals, there is also a second effective (ground fault) return path.  The ground fault path starts at the neutral buss, where the returning current divides.  Some current will return as intended, on the shore power neutral conductor, but some will divert to the shore power cord’s ground conductor, through the inverter’s as yet unbroken neutral-to-ground connection.  That diversion path is a true ground fault.  One half of the total current will flow in each path.  The pedestal ground fault sensor expects the outgoing and returning currents to balance (within 30mA), but in this case, that sensor will see much less current returning on the neutral conductor than what was delivered on the hot conductor.   The pedestal breaker will want to trip.  How fast will it take for the trip to happen?  Usually between 30mS (t=0.030) and 50mS (t=0.050), but in all cases, less than 100mS (t=0.100), the maximum specified for the pedestal circuit breaker to trip.

In any case, we now have a “race” condition.  The race “contestants” are 1) the inverter relay against 2) the ground fault sensor.  The intent is for the inverter relay to “win.”  My inverter’s spec for transfer time is 18mS (t=0.018).  But, if the time it takes for the shore power Ground Fault sensor to trip is less than the time it takes the inverter’s relay to transfer into “Standby” mode, the pedestal breaker will indeed trip.  Furthermore, turning the inverter “off” will not eliminate that ground fault condition because the inverter’s internal relay would still be de-energized (“down”), and therefore, even with the inverter set “off,” its internal neutral-to-ground bond would still be present, creating the ground fault path.  Regardless, if the neutrals are separated, no cross-connection, so “no problem!”  So yes, it really is necessary to separate the branch circuit neutrals of the inverter-fed circuits from the neutrals of circuits that are not fed from the inverter. Elimination of the cross-connection of these neutrals is what eliminates the unintended, unwanted ground fault path.

Although I have not implemented an Inverter Bypass Switch aboard Sanctuary, I have drawn up a circuit diagram for such a switch, for those interested.  In Figure 8, the bypass switch is shown in the “Bypass” position.

When in “Bypass,” the switch’s external AC “power in” (red lines) comes from the hot and neutral lines that also feed external AC to the inverter.  Note that the “hot” feed for the bypass switch is upstream of the inverter’s power switch on the “Shore 1” AC panel.  This arrangement allows for bypassing the inverter while at the same time enabling a service technician to apply AC power to the inverter for diagnostic testing and repair verification.

When planning for the installation of an inverter, two pre-purchase considerations are, 1) what branch circuits will be powered from the inverter, and 2) what does the capacity of the inverter need to be in order to support the load of those circuits?  Aboard Sanctuary, we determined that we wanted to have AC power in the galley and at other utility outlets while underway.  That allows us to use our coffee maker, microwave, toaster and crockpot (not all at the same time), keep our DVR and AC lighting active, and occasionally charge utility batteries for my power tools.  We selected a 2kW inverter/charger to do that, which provides a maximum continuous AC output of 15A, shared by our four utility branch circuits.  That has served us well for 12 years.

Following is a “cut ‘n paste” from my “project plan” for the installation of our UL-458 compliant inverter/inverter-charger into Sanctuary’s DC and AC electrical systems, and timeframes based on my personal DIY-install timeline.  My need to “reconfigure” the B+ and B- DC busses on Sanctuary was because I consolidated the batteries from two separate banks (“house” and “start”) into a single bank at the same time, and updated the battery monitor from a stand-alone Xantrex monitor to a Magnum BMK. Combining banks greatly simplified battery charging from both the inverter/charger and the engine alternator.  Those steps are not specifically necessary for the inverter installation, but I like the consolidated battery bank.  Click to see my article describing that change.

Harmonic Distortion of AC Power

Initial post: 6/7/202
Minor edits: 6/8/2020

I’m posting this here because it came up on a boating club Forum that I follow.  As I have said often, my “target audience” is people in boating that do not have much prior background in matters of electricity.  This topic is a bit arcane, and does tend to be an advanced topic.  But at the same time, it does show up as a symptom that affects some boaters in some situations, so I offer it here for awareness.

Here is the question that started the discussion:

– – – – – – – – – – – – – – – – – – – QUOTE – – – – – – – – – – – – – – – – – – – –

“I would like to elicit opinions from the electrically minded of us regarding the following.  When running my NL 9Kw gen at anchor my Dometic/Cruisair heat pumps (240V, 16000 btu) work fine with just one of my Magnum Energy MS2812 (2800W, 125A charger) active to charge the batteries. But, when the 2nd charger is activated (now balanced loads on the gen legs), the heat pump compressors stop active function (no heating/cooling), fan drops to minimum level, but, amp load is unchanged. The above occurs whether 1 or all 3 Dometic units are running (this is not about trying to start one of the compressor motors with the gen loaded).  I have not noted this interference when the battery charging load is minimal.  The gen amp output at 100% is 37.5/240V.  Max charger demand is 17A both legs.  All 3 heat pumps together draw 13-14A. There is no problem if the water heater is run (240V/10A) with the heat pumps on and just one charger (brief test – 40A on one leg).

“It seems as though there must be some type of electrical interference that is occurring when the 2nd charger is added to the circuit affecting the heat pump compressor motor function. Any ideas as to what this might be and how it can be tested for? Emails were sent to NL and Dometic with no response. Thanks!”

– – – – – – – – – – – – – – – – – – – END QUOTE – – – – – – – – – – – – – – – – – – – –

Here is my response to this question, edited for completeness, which I offer to others who may be experiencing similar intermittent, “weird” symptoms:

– – – – – – – – – – – – – – – – – – – START – – – – – – – – – – – – – – – – – – – –

What you are describing sounds like a somewhat out-of-the-ordinary (but not “extraordinary”) problem called Harmonic Distortion.  Here’s the electrical theory of HD in four sentences: A pure resistance – water heater heating element, light bulb, running motor – draws current in linear proportion to its impedance (according to Ohm’s Law).  Electronic devices do not follow Ohm’s Law;  they can and do draw current in short bursts within the AC sine wave voltage cycle.  These electronic devices are called “non-linear” loads.  Since in non-linear loads, current does not follow Ohm’s Law against voltage, the apparent internal impedance of the source can cause the waveshape of the AC voltage to distort (dip, flatten at the top and bottom), rather than be or remain a pure sine wave, clean as the driven snow.

So in your situation, the inverters are AC loads being used for battery charging, but the battery charger’s internal DC circuits are non-linear, “switch-mode” devices.  That creates non-linear current demand on the input AC waveform that is reflected back into the source.  The system doesn’t fail on shore power because the apparent impedance of the shore power source is many, many, many times less than the apparent impedance of the genset.  That doesn’t mean the phenomena isn’t there on shore power.  It just means the source is big enough to overcome the magnitude of the non-linear load component.  On shore power, the ratio of load impedance to source impedance is sort of analogous to David-on-Goliath.   But with the much smaller capacity of the genset, the aggregate effect of the switch-mode current demand can affect the shape of the genset’s output voltage sine wave.  Here, the ratio of load impedance to source impedance is definitely David-on-David.  What tends to happen with Harmonic Distortion is that the positive and negative peaks of the AC sine wave flatten, although more complex distortion is possible in extreme cases, even to the point of approaching a square wave with a flat top and very low peak voltage.

You mentioned in your post that you have a 9kW NL genset.  Nine kilowatts is somewhat under-sized for a 250V, 50A boat.  The power that can be absorbed by a 240V, 50A load is 12000 Watts, or 12 kW.  What you have is NOT “bad” from the perspective of genset loading or the perspective that you rarely need the entire capacity of the generator anyway.  But, if what you have is a symptom related to Harmonic Distortion, the smaller genset will have a higher apparent impedance than a larger genset would have.  The higher the apparent impedance of the source, the more likely it is that Harmonic Distortion would present itself as a noticeable and annoying symptom.

My conjecture that this is Harmonic Distortion is easily confirmed with an oscilloscope.  In the old days, that was the only way to see it.  But today, you can confirm it easily it if you have a means to read TRUE RMS voltage and a means to measure the TRUE PEAK voltage.  The peak of a 60Hz sine wave should be 1.414 times the RMS value.  I use an Ideal SureTest 61-164 or 61-165 circuit tester for this task.

So let’s assume you have a stable 60Hz voltage at 118V when running on the genset.  And we must also assume you have a stable 60hZ frequency, ±2 hZ, when running on the generator.  Multiply the 118 x 1.414, and the peak of the voltage waveform should be 167V.  If you then measure the actual peak, and it’s – let’s say – 156V, then you know you have Harmonic Distortion taking place, and the wave form isn’t a pure sine wave.

Now, the tolerance of the inverter/charger(s), the SMX Controller electronics and the blower drive electronics of the heat pump to AC voltage waveform shape, for which they, themselves, are responsible for distorting in the first place, may not be favorable.  That is a vicious circle.  It’s creating something that it, itself, can’t live with.  Since the genset is also feeding the Dometic SMX heat pump control unit and the blower and compressor control electronics of the heat pumps, those circuit boards can also be impacted by distortion of the voltage waveform.  Symptoms across the onboard system can be unpredictable, and can vary from attachment to attachment.  Pure resistance loads will not be affected, but electronic devices can be to varying extents.

Harmonic Distortion and Power Factor are two of the most challenging problems power utility companies have to manage.  A distorted AC voltage sine wave waveform is called “dirty power,” and it costs utilities a lot of money to manage.  Buildings with banks of computers and servers cause huge HD problems on the power grid, often affecting their neighbors and neighborhood.  Virtually all electronic devices cause Harmonic Distortion, right down to the family flat screen TV and stereo.  Power quality is a huge problem at the level of commercial power utilities serving residential neighborhoods.

And by the way, from the perspective of the 9kW NL generator itself, the higher apparent impedance and distorted wave shape will cause additional heat in the windings of the genset.  That heat is not related to useful work done by the generated power.  It amounts to excessive waste heat of which the genset’s cooling system has to dispose.  This can be worse than having unbalanced 120V loads on each side of the genset.

The fix?  You’d need a bigger capacity generator; i.e., one with lesser internal impedance.  With a lower reflected impedance, the genset would maintain the shape of the waveform for equivalent non-linear loads.  Or, your can just choose to live with it…

I have not written about Harmonic Distortion or Power Factor for my website because it’s definitely not a beginner’s/layman’s topic.  (Well, I have now, haven’t I?)  And even if you have HD, there’s little that can be practically done.  But if you want to read more about HD, click here for a fairly readable and reasonably good explanation from Pacific Gas & Electric; and click here for a better explanation of non-linear loads.  Start on page 3, at the heading called “ELECTRICAL HARMONICS.”  Skip the math; you don’t need it to understand the concepts.

Hope this helps.  And of course, this is only a guess on my part…   Cough, cough, choke, choke…

I wish I could recommend something practical that would make this better, but in the current system configuration, I think it’s a permanent restriction.

– – – – – – – – – – – – – – – – – – –  END  – – – – – – – – – – – – – – – – – – – –

Understanding Harmonic Distortion is complex and it’s definitely an advanced problem in an electrical distribution system.  What I’ve written above is just the very tip of the the technical iceberg.  But, although relatively rare, HD can produce observable symptoms related to the performance of boat AC electrical attachments.  It can affect the quality of sound from an entertainment system or produce what looks like interference (snow, lines) on a TV.  And, it can affect the operation of other types of equipment, like network routers, DVRs and printers.  If you have these symptoms and all else has been ruled out, consider Harmonic Distortion as a possible cause.  If you have these symptoms, it will be necessary to call in a skilled professional electrical technician to troubleshoot and confirm the diagnosis.  The tools that are necessary are expensive, and the skills to appreciate and understand the causes are advanced.  This is not a job for a residential electrician.

ABYC Electrical Standard Mapped to Sanctuary’s AC System

4/20/2020: Significant editorial updates to content.
5/27/2020: Added borders to images via HTML edits.

INTRODUCTION

All boaters at one time or another get involved in discussions about what boats “are required by standards and codes to have or to do.” This comes up every time the owner is faced with getting a boat survey. A boat survey report usually makes copious references to “ABYC Standards” and to “industry best practice.” But the vast majority of boat owners do not work in a world of industrial codes and standards and are not familiar with what they are, what they are intended to do, and how they are used throughout the marine and commercial business world (especially, the insurance risk world).

This article is in the form of a stand-up classroom presentation. Slides are presented along with text (“speaker notes”) that describes the slide’s content. This is a mix of “engineering” and “safety.” My hope is that this material will make sense in this format. What I do in this article is look at the “electrical system” of our own boat, and compare that to the requirements of the principle ABYC electrical standard, E11, “AC and DC Electrical Systems for Boats.”

Our trawler, Sanctuary, is a Monk36 Trawler fit with two 120V, 30A shore power service cords. In our case, the shore power cords are configured so that one feeds the house AC loads and the other feeds our heat pump AC loads. Many boats are configured in the same way, but other configurations are possible. Our house loads include a battery charger for our genset start battery, fridge, hot water heater, inverter/charger and several utility outlets. The heat pump loads include one 5kBTU self-contained unit and one 16kBTU self-contained unit and a raw water circulator pump.

While configurations of individual boat electrical systems may be different, the ABYC Electrical Standard E11, “AC and DC Electrical Systems on Boats,” applies equally to all electrical system configurations on all boats of all designs and hull forms. Boats that adhere to the ABYC electrical standard are highly likely to be safe and compatible with 2020 shore-side infrastructure (marinas, boatyards, community, condo, municipal and residential docks). These standards are intended to maximize the safety of the boat; safety from shock hazards, freedom from ground faults, freedom from accidental fire hazards and much worse. I strongly encourage boaters to bring their boats into compliance if that is not already done!

THE LAYOUT OF A BOAT ELECTRICAL PLATFORM

Figure 1 shows an “energy flow diagram” of the total electrical system of a typical cruising boat, comprised of three separate divisions. The central electrical system is the vessel’s DC division (shown in red). This is the division that starts the engine and powers navigation lights, pumps, windlass and miscellaneous navigation equipment. All engine-powered boats have DC systems, but AC divisions are optional. Sanctuary’s platform also has an AC division (shown in green) which allows captain and crew to enjoy the comforts of a shore-side residence. Interfacing between the DC and AC divisions is a means to charge the batteries, and optionally, also use the batteries to power all or part of the AC division.

2

Note: in this topology view, solar battery charging systems would be part of the DC Division.

Note: out-of-scope for this article is the Bonding System Division of the electrical system. Those interested are referred to my article “Bonding System Design and Evaluation.”

Figure 2 shows the interfacing division with an inverter/charger instead of a battery charger. The red highlighted lines show the Inverter/charger in “Invert” mode. For the inverter to be in “Invert” mode, no other AC power source is available to the vessel; ie, no shore power, and no onboard generator running. Absent a source of AC power, the inverter draws DC power from the batteries, converts it to AC, and provides AC power to a subset of AC circuits on the boat. This operating mode would be the typical operating mode for boats at anchor, or boats underway on a travel day. While at anchor or underway, power is available for an AC coffee maker, a microwave, a crockpot, AC space lighting and entertainment systems, and an AC charging source for computers, onboard routers, smart phones and tablet computers. At least, that’s what we do aboard Sanctuary.

3

In Figure 3, the red highlighted lines show the flow of AC power when the boat is connected to shore power via a dock-side pedestal. AC Power enters the boat at the SHORE POWER INLET, passes through a MAIN DISCONNECT BREAKER to, and through, the GENERATOR TRANSFER SWITCH and on to a DISTRIBUTION PANEL which supplies HOUSE LOADS. AC Shore Power passively “Passes Through” the INVERTER/CHARGER to power a subset of AC loads, and the inverter/charger device acts as a DC BATTERY CHARGER.

4
Note: in this topology view, the inverter/charger is fully integrated into the boat’s electrical system, and automatically switches between “Standby/Pass Through” mode and “Invert” mode as AC power from another source comes and goes. If a boater in a neighboring slip accidentally turns off Sanctuary’s pedestal breaker(s), our inverter/charger automatically transfers to “Invert” mode to maintain AC power to it’s attached loads. This configuration is the ONLY use case that ABYC supports for inverters or inverter/chargers installed aboard boats.

Figure 4 shows the above AC Electrical System components mapped to the actual wiring diagram detail of Sanctuary’s installed AC electrical system. The remainder of this article focuses on ABYC requirements of the E11 standard related to the AC Division of the boat platform.

5

Note: Sanctuary is not fit with an Isolation/Polarization transformer (shore power transformer). Shore power transformers have a number of unique ABYC requirements and considerations. Consult the E11 standard for the treatment of these devices.

Note: I occasionally hear that an isolation transformer has been recommended as a means of avoiding the need to “spend unnecessary money” in order to fix/correct conditions aboard a boat that cause dock-side ground fault sensors to trip AC shore power “off.” I strongly discourage that thinking. The conditions that cause ground fault sensors to trip are often serious, potentially dangerous electrical safety or fire hazards. Transformers do mask safety problems which can be a threat to the boat and its occupants, but they DO NOT CORRECT THE UNDERLYING ELECTRICAL FAULT-CAUSING CONDITIONS.

Figure 5 is a clear view of the wiring detail of our AC electrical system. Notice that the neutral buss for house circuits has been divided so that the circuits fed from the Inverter/charger are separated from the house circuits that are not. Further, except as necessary for explanation, AC safety ground wiring is not shown on this diagram; that is a conscious choice made in the interest of simplifying the diagram.

6

THE ABYC ELECTRICAL STANDARD, E11

The ABYC electrical standard is quite extensive and complex. This presentation only covers the major highlights that apply to the AC system division. Similar requirements apply to the DC division. Get these basics right and the boat will be well on its way to being safe. This presentation does not include a discussion of the requirements of onboard 120V load circuits; it focuses on the power distribution components of the AC division, to which we normally give little specific consideration.

L5-30

By far the most common 120V, 30A shore power connectors are National Electrical Manufacturers Association (NEMA) L5-30R and L5-30P pairs. These are found on the familiar 120V, 30A commercial cordsets. I do not like them because I feel they are not nearly robust enough for the repetitive removal and replacement to which shore power cords are subjected in normal use. NEMA L5-30 connectors were designed 80 years ago for light industrial applications where outlets were sometimes ceiling mounted and machinery cords hung from ceiling receptacles. They were plugged in and given a twist, and they were rarely touched again. They are not intended to be roughly handled by boat owners and dock assistants, dropped on docks, stepped-on, rained-on, snowed-in and otherwise abused in routine service.

Which brings up an important point about all ABYC standards. The “requirements” stated in ABYC E11 are MINIMUM PERFORMANCE REQUIREMENTS. They do not require a particular piece of equipment or a particular manufacturer’s product. They simply specify minimum compliance requirements. So, NEMA L5-30P/R connectors ARE NOT “required” by the standard. What is required is a “grounding plug that locks into place” so it can’t “fall apart.” Also realize, ABYC standards apply to boat manufacturers, marine equipment manufacturers, and service technicians. Only indirectly do they apply to boat owners. The standards DO NOT contemplate that DIY electrical work will be done by owners, but they do contemplate that all work done by anyone will comply with the requirements.

0DA35F75-5CF5-4F08-B340-40F94896936A_1_201_aI have personally chosen to replace the OEM NEMA L5-30P shore power inlet receptacles with those made by SmartPlug, LLC (http://www.smartplug.com/) (no personal financial interest; just a very happy customer). I personally feel SmartPlugs are much safer and more robust than L5-30 twistlocks, and they meet all NEC (UL, cUL, eTL) and ABYC requirements. That said, the SmartPlugs EXCEED the minimum performance requirements of the E11 standard.

The following slide shows requirements for the shore power CORDS and the shore power INLETS of the boat. The E11 standard refers to the “Type” of the wire. The cord’s “Type” descriptor is part of the information printed on (or molded into) the cord’s insulation, and should be easily readable on all marine-complaint cordsets. Don’t worry about the “Type” descriptor on Shore Power cables unless for some reason (I discourage this) doing a DIY shore power cord fabrication project. Simply buy products made by marine manufacturers and certified for marine use. The cordset manufacturers will have covered all that’s necessary for ABYC standards compliance.

7

The following slide illustrates a very important concept for shore power systems which all boaters should know; most especially, those who do DIY electrical projects!  At the head of the dock, in the facility’s electrical service infrastructure, the safety ground conductor is bonded (connected) to the neutral conductor. This is an NEC code requirement for all sources of AC power throughout North America, and results in a system referred to as a “Grounded Neutral System.” In a “Grounded Neutral System,” the neutral is intended to carry all of the current returning from the boat to the shore-side source. By design, the ground conductor IS NOT intended to carry current except to trip a circuit breaker in a fault situation. Thus, the neutral-to-ground bond is located in the facility’s infrastructure for both 120V and 240V systems.

8

The following slide emphasizes the boat-side of the shore power connection. The E11 standard requires that there be no neutral-to-ground bond(s) on the boat. At this point, for clarity, that firm statement can be modified to read, “there must be no neutral-to-ground bond(s) on the boat when operating on shore power.” The reason for this distinction now will become clear later, but for shore power, if there is a neutral-to-ground bond on the boat, that wrongly-placed bond creates a connection between the neutral conductor and the ground conductor that electrically parallels the two conductors all the way back to the dock-side infrastructure’s correct ground bond. Since the ground conductor on the boat is in direct contact with the sea water in which the boat is floating, this also parallels-in a ground path through the sea water. When all of these paths are in parallel, current that should flow only on the neutral will divide and flow in equal amounts on both conductors, and in some amount, through the water itself. By definition, this is a “ground fault,” and it will trip power “off” if there are ground fault sensors on the dock-side pedestal, but it can also kill people, pets and wildlife in the water. Incorrect neutral-to-ground bonds on boats are a primary cause for AC power leaking into the water, and can lead to incidents of ELECTRIC SHOCK DROWNING. For further information, readers are referred to my article on “Electric Shock Drowning.”

9

The following slide shows correct and incorrect wiring examples. In my article entitled “AC Electricity Fundamentals – Part 1,” I explain that a boat connected to a pedestal is intended to be wired like a sub-panel in a residential installation. Many residential electricians and DIY boat owners do not understand that technical detail, and so often connect neutrals and grounds together as they would in the main panel of a residence. On boats, as explained above, this is WRONG and DANGEROUS. Those who DIY must understand this natty technical detail.

10

The following slide shows the next major component in the flow of AC power into the boat: the Shore Power MAIN DISCONNECT BREAKER. This device is mainly for overload (and since 2012, ground fault) protection. Note that for 120V, 30A circuits, both the hot conductor and the neutral conductor must be switched, so this disconnect must be a 30A, “double-pole” circuit breaker with either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other side is also opened.

12

Boats built before 2012 will not have OEM ELCI (Equipment Leakage Circuit Interrupter) circuit breakers installed. That is OK. Although required since 2012 on new construction boats, ABYC states that boats that complied with the version of E11 that was in effect at the time the boat was built by the OEM manufacturer are “grandfathered” for compliance. Note that MANY MARINE SURVEYORS do not choose to adhere to/acknowledge the ABYC “grandfathering” policy. That can result in an inappropriate non-compliance finding in a boat survey.

The following slide shows the MAIN DISCONNECT SWITCH on a boat fit with 240V, 50A service.  The significant difference is that here, only the two hot conductors (L1 and L2) are switched. The neutral is not switched. Thus, a double-pole breaker rated at 50A is appropriate here. As before, this breaker must have either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other also opens.

13

Note that the neutral-to-ground bond is only correctly located in the shore power infrastructure, which is one of the National Electric Code (NEC) “rules” for residential and light commercial 120V/208V/240V electric services.

The following slide illustrates another very important wiring detail. Recall, Sanctuary is served by two 120V, 30A circuits. Earlier, we saw that neutrals and grounds MUST NOT be connected together aboard the boat. This is a similar case, and for the same reason. Here, it’s essential that the neutrals from Shore Power Circuit 1 and the neutrals from Shore Power Circuit 2 be SEPARATED aboard the boat. The reason is, both of the neutrals run back into the marina pedestal, or may run all the way back to the marina main service panel. If they are connected together on the boat, they become electrically paralleled all the way back to wherever they are ultimately joined together (pedestal junction, panel neutral buss, etc). All current returning from the boat will divide and flow equally on both neutrals. By definition, that is a “ground fault” at the pedestal circuit breakers, which will trip both breakers and interrupt power to the boat. But even more importantly, if one of the shore power cord neutral conductors were to fail open (due to, for example, a burned blade on a NEMA L5-30P twistlock plug), the other neutral circuit would become overloaded and could easily become a fire hazard aboard the boat. Preventing that fire hazard is why understanding and complying to these standards is important.

14

The following slide shows the “right” and “wrong” views described above.  Again, MANY, MANY  RESIDENTIAL ELECTRICIANS DO NOT UNDERSTAND THIS REQUIREMENT BECAUSE BOATS ARE NOT HANDLED IN THE SAME WAY AS THE MOST COMMON RESIDENTIAL INSTALLATIONS.

15

And by the way, the “wrong way” is a common way to find neutral wiring done on older boats.

Check your boat.

The following slide highlights the need for Equipment Leakage Circuit Interrupter (ELCI) devices for protecting against ground faults on the boat.

16

Those interested can read more about ELCI circuit breakers in my article entitled “ELCI Primer.”

The ELCI requirement was added to ABYC E11 in 2012 for new boats. ELCI devices are intended to both protect from overloads and detect ground faults. Ground faults on boats can result in dangerous levels of AC power being dumped into the water, which is a hazard that can lead to Electric Shock Drowning (ESD), as discussed previously.

An ELCI device on the boat is the same thing as a “ground fault sensor” on the dock-side pedestal (ground fault sensors on docks have many acronyms, including “EPD,” “GFD,” “GPD,” and “RCD;” don’t worry about what they’re called. By any name, they do the same thing.) ELCI devices also do the same thing as pedestal sensors, but the ELCI is physically installed aboard the boat. The value of having an ELCI on the boat is twofold. First, the simple act of installing an ELCI will flush out any silent, hidden wiring problems that currently exist on the boat. Second, ELCI will trip instantly upon the spontaneous emergence of a ground fault issue on the boat at some later date, so the boat owner will become aware of it, and be able to initiate repairs, as soon as it surfaces as a safety issue.

The following slide introduces the concept of a GALVANIC ISOLATOR. Galvanic Isolators are very important to controlling corrosion of underwater metals on any boat.

17

Galvanic Isolators are installed IN SERIES WITH the safety ground conductor AT THE POINT WHERE THE GROUND CONDUCTOR ENTERS/EXITS THE BOAT. Nothing – NOTHING – should be connected to the side of the isolator that leads to the shore power inlet connection except the actual safety ground conductor, itself.

18

The E11 standard considers Galvanic Isolators to be “optional” equipment, but if they are installed, the standard provides installation requirements.

If a Galvanic Isolator is NOT installed, the rest of the GROUNDING CONNECTIONS are still mandatory.

19

Earlier, above, the ABYC requirement that “there must be no neutral-to-ground bond on the boat when connected to shore power;” was mentioned with the proviso that it would “become clear later.” Now is the time to clarify as we look at the topic of POWER-SOURCE SWITCHING. The following slide shows the three possible sources of AC power on Sanctuary: 1) shore power, 2) genset, and 3) Inverter. The North American design standard for ALL AC power sources is, ALL power source neutrals are grounded at the source. Since shore power sources are grounded on land in the facility infrastructure and NOT aboard the boat, and since both the generator and the inverter are located aboard the boat, then how is it possible for them to be “grounded at the source” if neutral-to-ground connections are not allowed on the boat? Well, compliance is accomplished through appropriate source transfer switching.

20

Note the construction of the GENERATOR TRANSFER SWITCH shown on this slide.  That Generator Transfer Switch on Sanctuary is a three position rotary switch: “Shore,” “Off,” “Generator.” When the switch is in the “Shore” position, the generator’s neutral-to-ground bond is switched out of the circuit, thus meeting the shore power separation requirement. When the switch is in the “Generator” position, the shore power circuit is switched out of the boat’s electrical platform, thus permitting the onboard neutral-to-ground bond at the generator. The same type of logical switching is accomplished for the inverter by a relay located within the inverter.

Note: ABYC A31 requires that Inverters installed on boats be certified to UL458 (Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts) to ensure this grounding management relay is present. ABYC E11 includes ABYC A31, amongst other boat electrical standards. BOAT OWNERS SHOULD ENSURE THAT ANY INVERTER INSTALLED ON A BOAT IS COMPLIANT WITH UL458. Especially, be aware that inverters from Harbor Freight and other discount sources will not be compliant to UL458 and are not suitable for use on mobile platforms like boats and RVs.

Following is a close-up of Sanctuary’s Generator Transfer Switch. This is a three-position rotary switch. There are other switching styles that use lockout slide mechanisms to accomplish the same thing. Here, the breaking of the neutral conductors is highlighted by the red ellipses.

21

The following slide is just a reminder of what we looked at earlier WITH RESPECT TO SHORE POWER SOURCES. For Shore Power, the neutral-to-ground bond is in the shore power infrastructure and NEVER on the boat.

22

And this slide shows the generator neutral-to-ground bond that is switched into the boat’s onboard AC system when the genset is running…

23

And this slide shows requirements specific to inverters…

24

The following slide moves to another very important safety issue. Rarely, it is possible to encounter a 120V dock power pedestal source in which the black (hot) and white (neutral) wires (or red and white) are physically reversed inside the pedestal or other location in the dock-side infrastructure. No, it should not happen. Yes, it should be found by the installing electrician before the circuit is put in service. But folks, it does happen (rarely, thankfully). I have seen it three times in 16 years of cruising.

25

What’s particularly bad about a “reverse polarity” situation is that it can be present and also be entirely symptomless on the boat. Electrical equipment aboard the boat will work normally. But, touch potential shock hazards are likely. Because this condition is largely symptomless, it’s important to detect it and warn the boat operator of the potential life-safety issue. The “RP” warning lights (and/or audible alarms) are connected between the Safety Ground (green) and the Neutral (white) conductors on the boat. There should never normally be more than a volt or two between those conductors. Anyone who sees a ”Reverse Polarity” warning light(s) illuminated on their boat should immediately DISCONNECT (physically unplug) the shore power cord from the pedestal and report the condition to facility management. This can be a potentially lethal condition in the right (wrong!) circumstances.

26

This slide shows “Reverse Polarity” warning lights wired between the safety ground and neutral conductors aboard Sanctuary.

Actually, Sanctuary has some duplication here. Our Generator Transfer Switch has Reverse Polarity indicators, as do both shore power distribution panels.
ABYC specifies the minimum impedance of RP detection devices must be ≥ 25kΩ. Since these devices are connected between the neutral and the safety ground, they are a possible path for small “ground fault” currents, and properly installed sensors on some boats can cause false trips of a dock-side or ELCI ground fault sensing device. This would be caused by either multiple sensors in combination or older incandescent sensors having too low an impedance, thus allowing too high a “ground fault leakage current.”
SUMMARY

The most current revision of the ABYC E11 Standard (July, 2018) as of this writing (April, 2020) is 67 pages of “shall” and “shall not” requirements, technical tables and example electrical drawings. Far more than I have covered here. Furthermore, there are several other ABYC Standards that apply to electrical subjects, such as A27, “Alternating Current Generators,” A28, “Galvanic Isolators,” A31, “Battery Chargers and Inverters,” E2, “Cathodic Protection” and E10, “Storage Batteries.”

These standards – and all ABYC Standards – make us all safer. They save property damage losses and they save lives. A marina fire is one of the most terrifying things any boater can ever experience, and there have been several this year alone (winter, 2019-20). When we aboard Sanctuary arrive at a marina, we must assume all of the boats that will be our new dock neighbors are safe. All of those boaters must also assume that we are safe. These standards are the reason we can all have some confidence in those forced assumptions. If there are condition(s) aboard your boat that you know need to come into compliance, please do so. The family you save may be your own!

For the record, I’m not much of a fan of covered slips, either. Those roofs help with UV damage and weather, but in a fire, heat arising from the fire’s origin is contained by the cover and spreads linearly along the dock until the cover finally burns through. This greatly foreshortens escape time; and, not a good thing for survivability of boats that were otherwise uninvolved in the first place. Always think fire safety and escape routes…

Electrical Behavior of a 208V/240V Boat

This article discusses the electrical behavior of the two 120V AC circuits on a boat that is natively wired for 125V/250V, 50A shore power service.  Topics include current flow (Amps) in the different appliance loads, power limitations when connected through a “Smart Splitter,” and the constraints and limitations encountered with the use of certain shore power transformers when powered from 208V dock utility voltages.

Use Case 1: a boat wired with a 125V/250V, 50A shore power cord, but not fit with 240V appliance loads.

Figure 1 is a generic wiring diagram illustrating this use case.  The system includes a genset and a Galvanic Isolator.  In Figure 1, the dock power source is on the far left and the boat’s appliance loads are on the far right.  Dockside 50A circuit breakers are omitted for simplicity.  The 50A shore power cord is highlighted in the red oval.  One 120V load (the heat pump) is highlighted in red.  Other 120V loads (house loads) are shown in black.  This boat DOES NOT have 240V loads.  This use case is a very common “50A” boat configuration.

Use Case 2: a boat wired with a 125V/250V, 50A shore power cord adapted to two 120V, 30A pedestal outlets to obtain limited 208V/240V power.

Figure 2 is a generic wiring diagram illustrating this use case.  Most commonly, a “Smart Wye” splitter adapter is used (ref: Appendix 1).  A “Smart Wye” splitter has two 30A twistlock plugs (NEMA L5-30P) and one 50A receptacle (NEMA SS2).  The two 30A receptacles (NEMA L5-30R) are on the dock pedestal.  The splitter and the 3-pole, 4-wire, 50A power cord are shown in the red ovals.  The rest of this system is identical to Figure 1.

Figure 3 applies to both Use Case 1 and Use Case 2 configurations.  Figure 3 shows logical blocks instead of actual circuit detail in order to make it easier to visualize the electrical behavior in this AC system.  In Figure 3, incoming power is shown as being derived from “any suitable 240V source.”  Electrically, we really don’t care how we get shore power as long as it’s “3-pole, 4-wire” of the right voltages.  In Figures 1 and 2, the loads were shown as they are wired, but Figure 3 shows them as they are logically arranged in the overall electrical circuit.  As the drawing shows, the red-highlighted 125V, L2 heat pump load is connected in series with the black-highlighted 125V, L1 appliance loads.  These two load groups share a common “Neutral” conductor.  The Neutral conductor anchors and maintains the midpoint voltage of the series connection under varying demand conditions.

Visualizing this electrical configuration in the mind’s eye as two 120V loads connected in series across a 240V source is the first key concept in this article.

Having identified the electrical arrangement of the two 120V appliance load groups of this 240V system, further analysis is on a) the voltages present, b) current flows, and c) power available to do work.

Figure 4 shows the two series load components of this boat’s 240V boat system, each with 120V across them.  The L2 load group is comprised of the boat’s heat pump(s) and raw water circulator.  The L1 load group is comprised of the hot water heater, fridge, battery charger(s) and multiple utility outlets.  Measuring across the L2 load between points A and B, there are 120V.  Measuring across the L1 load between points B and C, there are 120V.  The series pair receive the 240V mains supply voltage measured between points A and C.

Next, consider the electrical currents (measured in Amps) flowing through the two series load groups in a variety of specific but different load circumstances.  Understand that in the following analyses, different specific devices are “on” and others are “off” at any specific point in time.  Assume the following scenario: the boat’s owners have been away from their boat for a mid-summer week.  Upon late day arrival at the boat, outside air temperatures are in the mid-to-high 80s with 85% relative humidity.  Our boat owners will turn on some space lighting, and will immediately turn on the heat pump for air conditioning.  They will turn on the hot water heater and battery charger, stow fresh veggies, ice cream and adult beverages into the fridge, and perhaps turn on the DVR/TV.

Electrically, assume the heat pump draws 20A.  Also assume that house loads (hot water heater, battery charger, fridge, space lighting, computers and DVR/TV) add up to drawing 20A.

In Figure 5, the heavy red line represents this 20A flow of current (Amps).  This example is a special case called a “balanced-load” condition; that is, both of the 120V loads just happen to draw the same amount of current (20A).  The Amps flow from the dock pedestal into the loads on one of the energized line legs (L1), and flow back to the pedestal on the other energized line leg (L2).  In this balanced-load condition, no current flows in the neutral conductor (N).

Very importantly, notice that no more than 20A is flowing anywhere in this system. A double-pole 30A circuit breaker that serves the boat via a Smart Splitter at the dock pedestal sees 20A on both legs, L1 and L2.  Since there is no place in the system carrying more than 20A, the 30A pedestal circuit breaker is perfectly happy.  The second extremely key concept to take from this article is that the 20A flowing to power the heat pump circuit is the same 20A that flows through the House circuits to power the water heater, battery charger, fridge and utility outlets.

The word “power” is highlighted above to make the point that the same 20A flowing in the two 120V loads does useful work in both 120V load groups.  The basic formula for “Power” is P = Volts x Amps.  So in the heat pump load group, we have 120V * 20A = 2400 Watts.  In the house appliance load group, we also have 120V * 20A = 2400 Watts of power doing useful work.  In total, we have 4800 Watts of work being done at this time, in this system.

Up to 30A is available from a 30A shore power pedestal without exceeding the capacity of the circuit breakers.  The maximum power possible for each load is 120 * 30 = 3600 Watts.  Because the two load groups are in series, the maximum work that can be done by 30A, in total, is 7200 Watts.  If the boat had access to its design maximum of 125V/240V, 50A shore power, there would be the potential for 240 * 50 = 12000 Watts, total.  It quickly becomes clear why careful load management is necessary when running with two 30A cords feeding a 50A boat through a 30A Smart Splitter.

Following from our earlier scenario, after an hour or so, the hot water heater has done its water heating work, the fridge has done its cooling/freezing work, and the batteries are fully charged.  But, the heat pumps are still running to cool the boat.  Now, although we have 20A flowing in the heat pump load, current on the house side has dropped to 4A for the DVR/TV and space lighting.  Figure 6 shows what happens electrically.

The heavy red line represents the 20A needed by the heat pump.  But this time, there are only 4A needed by the house, represented by the thin red line continuing through the House circuit.  There is no longer a balanced-load.  The arithmetic difference between the heat pump demand and the house demand is 16A.  That 16A returns to the pedestal in the system’s neutral (N) conductor.  In this example, as before, there are 120 * 20 = 2400 Watts of work being done in the heat pump load group, and 120 * 4 = 480 Watts of work being done in the House load group.  There are never more than 20A flowing in any part of this system.  Neither the shore power pedestal breakers nor the Neutral conductor are overloaded.  All is safe and well within specifications.

At the end of the evening, when our sample boaters retire to bed, assume they turn off all of the house loads.  The hot water heater is satisfied, the battery charger is satisfied, the fridge is satisfied, the TV is “off,” the laptop and iGadget batteries are charged (and the screens have gone “dark”), and the space lighting is “off.”  Now, there is no current at all flowing in the House loads.  Ah, yes, but the air conditioning is still needed.

Figure 7 represents the electrical status in this case.  Since the heat pumps are still running, there are 20A flowing in the heat pump circuit.  Since there is nothing “on” in the House load group, the arithmetic difference is 20 amps, which returns on the neutral (N) conductor.  Again, no part of the circuit carries more than a total of 20A.

 

 

 

Use Case 3: a boat wired with a 125V/250V, 50A shore power cord, but fit with 240V appliance loads aboard.

Figure 8 shows the addition of pure 240V loads at the far right of the drawing.  Boats with 125V/250V, 50A shore power service which have both 120V and 240V appliance loads (hot water heater, cooktop, electric dryer, heat pump compressor) are electrically very similar to those without 240V appliances.  Very few “240V appliances” are “pure” 240V devices.  The only ones that come to mind are 2-pole, 240V deep well pumps and 2-pole, 240V hot water heaters.  Appliances like heat pumps, cook tops, ovens, clothes dryers and watermakers, are usually “hybrid devices;” ie, they need both 120V and 240V to operate.  The control circuits in hybrid appliances are generally 120V circuits.  In a dryer, for example, the heating elements are 240V but the motor that turns the drum and the clock timer circuit both require 120V.  Hot water heaters can be pure 240V-only loads which do not need or have a neutral conductor.

In Figure 8, the pure 240V appliance loads are electrically in parallel with the two 120V series loads, and the 240V loads add to the amps drawn in the 120V supply mains, L1 and L2.  So, if we had the 20A L2 load running a 120V heat pump, as has been the example throughout this article, and in addition, a 240V hot water heater simultaneously calling for 12A, the result would be a 32A total Amps in L2.  Attached to a 50A pedestal, all would be OK, but attached to a 30A splitter, the result would be a tripped 30A pedestal circuit breaker.  So again for emphasis, it is up to the boat owner/operator to understand load management and ensure that pedestal breaker capacity is not exceeded.

Potential Power Issues with Certain Shore Power Transformers

The utility power on docks can originate from two kinds of public utility sources.  “Single phase” sources will appear as conventional 120V/240V.  “Three phase” sources will appear as 120V/208V.  Because this electrical fact is a well-understood, and very common in boating, UL Marine certified electrical appliances are designed to accommodate the difference between 240V and 208V.  Residential appliances MAY NOT have have that same flexibility.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for  125V/250V, 50A applications   are manufactured in three “flavors:”

  1. Basic, single input, single output, 240V transformer; least expensive flavor.
  2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input to achieve a constant 240V output.
  3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.   While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.   Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”   The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.    The output of this transformer (the secondary) provides single phase, 3-pole, 4-wire power to the boat. In English, that means there is a conventional black, red, white and green output.    If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.   But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances.   With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances.  Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.  Microwaves will run but will take slightly longer to cook.   Coffee pots will perk, but will take slightly longer to perk.    Electric blankets will keep sleepers warm and cozy.   Water Heaters will heat water, but take slightly longer to reach target temperature.   Stovetop burners will heat, but not get as hot at the same setting.  Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage.  Marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.   One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V should be designed to detect the condition, put up a power warning fault light, and self-disconnect.    Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer marine heat pump designs do that.

WARNING:  if there is a 240V shore power supply voltage applied to a manual transformer set to a 208V input voltage, then the AC voltages aboard can get high enough to “damage” appliances.

Article Summary:

  1. When operating a 125V/250V, 50A boat which does not have 240V loads, total loads of up to 2 * 3600 Watts can be supported with two conventional 30A pedestal outlets.  In this case, neither the energized (hot) conductors nor the Neutral conductor are ever overloaded.  No individual circuit conductor ever conducts more than 30A.
  2. When operating a boat with pure 240V loads, the Amps required by the 240V loads add to the Amps needed in the 120V loads.  The owner/operator must monitor total amps drawn/power used to keep total power consumed below 3600 Watts per side.
  3. Some shore power circuit breakers are housed in inaccessible, locked locations ashore.  If a boater accidentally trips a shore power circuit breaker, particularly after hours, it may not be possible to gain access to it in order to reset it
  4. It is necessary for boat owner’s to closely monitor power usage and limit the amount of  current used to prevent tripping shore power circuit breakers.  Care must be exercised to not run high amp draw appliances (coffee pots, microwave ovens, inductive cookware, hair dryers, clothes washer/dryers and similar devices) at the same time.  Boats with multiple heat pumps will probably be unable to run all of them at the same time on 30A services.
  5. The examples in this article assume that the heat pump circuit is on one 30A load leg and house loads are on the other leg.  Obviously, some boats are wired differently. Systems with heat pumps and house loads distributed across both incoming energized 120V legs will have to monitor loads and current draws in the same manner, but the electrical principles discussed above remain the same.
  6. The specific balance of currents in the load one group and the load two group changes constantly.  L1, L2 and Neutral current (Amps) never exceeds 30A.

Appendix 1:

To the right is the electrical diagram of a typical “Smart Wye” splitter.  This Figure represents the electrical circuit detail of the splitter shown in Figure 2 in the earlier text.  Note that the splitter contains a relay – labeled “K” in the drawing.  The relay requires 208V or 240V to close.  Without at least 208V, the relay will not close and the splitter will not pass any power through to the boat.

Following is a link to my article describing Smart Splitters, and the receptacles required for their successful operation.

AC Electricity Fundamentals – Part 2: The Boat AC Electric System

Article posted: April 20, 2019
Added content: Shore Power Transformers; July 22, 2019
Added content: DC Electric Circuit “Wire” Naming and Identification; October 6, 2020;
Minor edits: October 6, 2020
Minor edits: February 16, 2021

Updated Diagrams: April 18, 2021

About this article

The AC Electricity Fundamentals – Part 1 article precedes this article and discusses 1) the concepts, terminology, components and layout of National Power Grid generating equipment, 2) delivery of AC power into residential neighborhoods, and 3) the configuration of AC electrical systems within a residential building, all at an introductory level. Boats attach to marina shore power systems.  The shore power system is the “host system,” and boats attach as “appliances.” An understanding of residential AC power systems is foundational to an understanding of AC power systems on boats.

The Part 1 article concluded by showing that the AC shore power system on boats is equivalent to a sub-panel in a terrestrial building. In the NEC architecture of terrestrial AC building systems, sub-panels are subordinate to the main service entrance panel of the building. In the same way, boats are subordinate to the shore power AC electrical infrastructure of a terrestrial facility.

This article focuses on the overall AC electric “platform” aboard cruising boats. On boats, shore power is only one component of a typical AC system “platform,” which can also include a mix of onboard generator(s), inverter(s) and in some cases, shore power transformer(s). This introductory Part 2 article will answer some questions, and will undoubtedly raise others. The goal of this article is to help readers understand AC electrical concepts and topics to be able to discuss questions, concerns, symptoms and options with marine-certified, professional electrical technicians.

Personal Safety

Virtually all electricity can be dangerous to property and life. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard. This is especially true of inverter-chargers. The large batteries found on boats can produce explosive gasses and store enough energy to start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! Anyone working in noisy environments, with running engines or other loud machinery, MUST WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!

Electrocution

Electrocution is a biological insult arising from an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death. Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electric shock or electrocution, in any locale around boats or water, there are several things that need to be done immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM! You can not save someone else if you panic!
  2. Avoid becoming a victim yourself!  DO NOT TOUCH THE VICTIM, METAL MACHINERY OR NEARBY METAL OBJECTS IF POWER IS STILL PRESENT!
  3. SCREAM FOR HELP! ATTRACT ATTENTION! Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  4. REMOVE THE POWER SOURCE FROM THE VICTIM BY DISCONNECTING THE ELECTRIC POWER at the pedestal.
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  6. THROW LIFE RING TO VICTIM. DO NOT ENTER THE WATER YOURSELF!
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again! Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate. CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.
  10. CONTINUE CPR UNTIL THE VICTIM REVIVES, UNTIL EMS ARRIVES TO RELIEVE YOU, OR UNTIL YOU ARE PHYSICALLY UNABLE TO CONTINUE!

Boat Electrical System – Scope

Viewing the shore power AC system of a boat as a residential sub-panel in a single family residence is simple and technically accurate, but the AC electrical system of a typical cruising boat is more than just shore power. A simple block diagram of a boat electrical platform can show the relationships among the various components of the boat’s AC (and DC) electrical systems. Sanctuary’s energy flow diagram is shown in Figure 1. This diagram shows Sanctuary as she is today. When we bought her, she did not have a genset, she did not have an inverter-charger, and she was fit with two inappropriate battery banks. She was less complex yet poorly designed for our intended use.

Figure 1

Figure 1 shows Sanctuary’s AC and DC electrical systems as a complete and integrated operational “platform.” From the platform perspective, owners can evaluate the impacts of contemplated alterations and upgrades. The diagram shows energy flow, not wiring detail.   Its simplicity allows one to visualize and understand both individual components and how components feed and are fed by one another. It is all too easy to add, remove and change system components without fully appreciating the impact(s) to the overall host electrical platform.

Sanctuary’s OEM factory configuration consisted of two 8D batteries, one dedicated to engine starting, and the other dedicated to modest OEM space and navigation lighting loads. An 8D was excessive and poorly utilized for engine starting, and inadequate for our house needs. The energy flow diagram gave us the ability to visualize the impacts of consolidating the two separate battery banks into a single bank.

Sanctuary’s OEM factory battery charger was an obsolete technology single-stage unit. We wanted AC power aboard without having to run our genset. We decided to change the battery charger to a fully automatic inverter-charger. This was a major upgrade that affected both the AC and DC electrical systems aboard. Mechanically, the upgrade was simple, but modification of branch circuit wiring to comply with the ABYC electrical standard was a big impact to our host AC electrical system.

Adding a new genset to a boat includes adding a Generator Transfer Switch and reworking the distribution wiring of the existing shore power circuits. Replacing an old-iron 60Hz AC genset with a new 60Hz AC genset would be relatively easy and non-disruptive. Converting to a DC genset (a diesel-driven DC battery charger) has very different implications. Cost, technical complexity and value of the alternatives can be compared and evaluated.

The energy flow diagram shows that Sanctuary is now fit with a single battery bank for both “engine start” and “house” support. Adding a wind generator or adding solar panels are energy management solutions that would have technical impacts to the existing system. Each can be evaluated from the perspective of our energy flow diagram. Boat owners are strongly encouraged to take a “platform view” as opposed to a “component view” of the electrical systems on their boats.

High Complexity Aboard Boats – Power Sources

Screen Shot 2021-04-18 at 09.21.04

What is immediately clear from Sanctuary’s energy flow diagram is that there are three entirely independent AC sources that can feed power to our onboard AC loads:
1. shore power (source ashore),
2. generator (source aboard),
3. inverter, (source aboard) and on some boats,
4. shore power transformers (source aboard) (not installed aboard Sanctuary.

DC Electric Circuit “Wire” Naming and Identification

In DC circuits on boats, the conductor carrying the positive charge is called “B+,” and also called the “plus” or “positive” conductor. By conventional agreement, the positive DC conductor is red in color. The conductor that returns current from the load to the source is called the “B-,” or “negative” conductor. By conventional agreement, the negative conductor (in 2020) is yellow in color. Until recent years, DC negative conductors on boats had black insulation, and many such systems are still in service today. In boats with both DC and AC systems installed, the black DC negative wire was easily confused with the black AC energized wire, so the ABYC color code for DC wiring was changed to yellow to eliminate the safety implications of confusing those two wires. In DC situations, the “negative,” or “B-” conductor is sometimes referred to as a “ground,” although that is usually (often) not technically correct, since “ground” wires are not intended to carry current. 

Key points from the Part 1 article to keep in mind on boats: In AC circuits, the conductors are alternately positive and negative, so the DC nomenclature “B+” and “B-” doesn’t work.  In single phase 120V circuits in North America, the two conductors are named for on their role in the circuit.  The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor.  By code and convention in North America, “L1” is black in color.  The other conductor in a 120V circuit is the return conductor.  It is called the “Grounded Conductor,” or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.  

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.”  They are commonly called “Line 1” and “Line 2.”  “L1” is black, and “L2” is red.  In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

Subtle take-away: the DC “negative” conductor has the same role in a DC circuit the the AC “neutral” conductor has in a residential/boat AC circuit.  That is, the DC “Negative” conductor returns current to the source.  In a “grounded DC electrical system” (very rare) the B- conductor is a “Current-Carrying”, “Grounded Conductor.”

Note: on some but not all boats built overseas, AC wire colors may be different than the ABYC Standard colors cited above. On some boats, like some Grand Banks trawlers, one 120V “hot” conductor (L1) is black, but the other (L2) is brown, not red; and the AC neutral conductors are blue, not white.  If “strange colors” are found aboard a boat, BE PARTICULARLY CAREFUL to determine what the colors mean to ensure ongoing equipment, fire and personal safety.  

Key Electrical Concepts For AC Services aboard Boats

Key points from the Part 1 article to keep in mind on boats:

  1. “Shore power” arises from the electrical system of a terrestrial facility, ashore, while AC power from a “generator,” “inverter,” and/or “shore power transformer” arises from equipment installed aboard the boat.
  2. The residential AC power standard in North America is a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system. This definition broadly applies to all terrestrial buildings with which people interact, and includes boats.
  3. State/Province, county and municipal jurisdictions across North America adopt local statutes and codes-of-regulations that originate with the NEC/CSA to govern terrestrial building electrical installations.
  4. There are no statutory electrical codes for boats. The American Boat and Yacht Council (ABYC) provides voluntary standards to boat builders. ABYC electrical standards are fully compatible with NEC shore power, assuring safe, reliable inter-operability between terrestrial and boat-resident AC systems.

As discussed in the Part 1 article, an essential safety requirement of all of these standards and codes is that single phase electrical systems be “grounded” at their “derived source.” This brings us face-to-face with some core ABYC “recommendations” that govern switching of AC wiring for equipment installations on boats:

  1. only one source is allowed to power loads aboard boats at any one time,
  2. sources must be thoroughly and completely isolated from one another,
  3. a “grounded neutral system” is required:
    • when on shore power, the neutral-to-ground connection is provided to the boat through the shore power cord, (i.e., the neutral-to-ground connection is in the shore power infrastructure), and
    • when on generator or inverter power, or when shore power is received through an onboard shore power transformer, the neutral-to-ground connection is made at the onboard source.

High Complexity Aboard Boats – Ground

In a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system, what does “grounded neutral” mean? Recall in the residential AC system model that three conductors arise from the utility power transformer at the street; two energized lines (“L1” and “L2”) and one neutral line (“N”). As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V between “L1” and “N” and between “L2” and “N,” but these voltages “float” with respect to their external environmental surroundings (recall the discussion of birds and squirrels on wires from Part 1). This situation is referred to as a “floating neutral.” To create a safe, known zero-volt system reference, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. The result is an earth-ground “grounded neutral” system.  The ground rod(s) reference the building neutral conductor to a very large mass at a zero-volt electrical potential (the Earth).  This Earth Ground IS NOT the same as “circuit ground.”  It would be the exception for a direct connection between L1 or L2 and the building’s ground rods to cause a circuit breaker to trip.  That is the function of “Equipment Bonding Conductors,” NOT the ground connection.  See my article on “Earthing and Grounding” on this website for more detail.

Grounding the neutral is very straight-forward at buildings. Since there is only one place where utility power enters the building from the utility company’s electric meter, it’s easy to understand and visualize that entrance location as the “derived source” of the power. Electrician’s working in terrestrial buildings learn to mix neutral and safety ground conductors on the same buss bars in the main service panel. In one of the examples I showed in the Part 1 article, we saw that some main service panels are built with only one buss bar which serves to collect both neutrals and grounds.

Boats are different!  In the architecture of the North American power framework, boats are sub-panels to the shore power infrastructure, not main service panels. Furthermore, it is common to have more than one AC power source for the AC system platform on a boat, including as we saw in Figure 2, AC Shore Power connections, onboard generators, inverters or inverter-chargers, and maybe shore power transformers (isolation transformer, polarization transformer). All of these sources are AC “derived sources” within the definitions of the ABYC Electrical Standard, E-11.

The NEC requires the neutral-to-ground bond to be at the “newly derived source” of the terrestrial shore power system. The ABYC electrical standard complements and supports the NEC requirement for boats operating on shore power. For boats operating on shore power, neutral-to-ground connections are not permitted aboard the boat. Why? Follow this scenario:

  1. Start with the NEC-required neutral-to-ground bond being correctly installed at the terrestrial facility’s main service panel (derived source) .
  2. The shore power neutral-to-ground bond is carried aboard the boat via the shore power cord, per ABYC E-11,
  3. The intent of the safety ground is to provide a low resistance electrical path to disconnect power as close as possible to the source in an electrical emergency:
    • in a normal AC system, no power flows in the safety ground conductor,
    • but in the case of an electrical fault, current flows in the safety ground for the purpose of removing power (fault removal) by tripping the circuit breaker that feeds the faulting circuit),
  4. Because there is a neutral-to-ground bond in the shore power main service panel, if there were also a neutral-to-ground bond aboard the boat, the neutral and ground conductors between the shore infrastructure and the boat would be electrically in parallel with each other, enabling power to flow in the safety ground (by definition, a ground fault). This results in two issues for boaters:
    • constantly trips a dockside ground fault sensing circuit breaker, and
    • the AC safety ground would, itself, be energized, thus providing a path to the underwater metals of the boat, thus enabling AC power to escape the boat’s electrical system into the water.
  5. The above consequences of paralleling the neutral and the safety ground pose a shock and electrocution threat to people, pets and wildlife in the water.

So, now we understand why a neutral-to-ground bond is not permitted aboard the boat when connected to shore power. But, we also know that ABYC does require a neutral-to-ground bond for onboard generators, inverters operating in “invert” mode and shore power transformers; that is, ABYC requires a grounded-neutral AC system throughout the boat regardless of the source of AC power.

Summarizing the above:  Shore power can’t have a neutral-to-ground bond aboard the boat, but generators and inverters must have neutral-to-ground bonds at the respective equipment aboard the boat. Isn’t this an irreconcilable “Catch-22?” In a word, “no!” It is a complex wiring situation that does not occur in terrestrial buildings where only one power source is present. (It does apply in terrestrial buildings if an outdoor emergency generator installed, and it also occurs in terrestrial off-grid solar applications.)

The technical solution that allows compliance with these apparently self-contradictory ABYC configuration requirements involves complex switching solutions. When connected to shore power, onboard neutral-to-ground bond connections must be “switched out.” When running on an onboard generator or an inverter in “invert” mode, the neutral-to-ground bond connection must be “switched in.”

High Complexity Aboard Boats – Switching

Marine-certified AC disconnect circuit breakers are readily available in a variety of form factors to fit different power panels of different companies found on different boats. With 120VAC, 30A inlet circuits, “Double Pole” breakers disconnect the “L1” and “N” lines. With 240VAC, 50A inlet circuits, “Double Pole” breakers disconnect “L1 and “L2,” but not “N”. It is up to the installing electrical technician to ensure that the correct disconnect breakers are used in the correct application to maintain compliance with the ABYC electrical standard and compatibility with the shore power infrastructure.

Looking at Sanctuary’s energy flow diagram, Figure 1, we can see that the boat’s Generator Transfer Switch (GTS) is used to transfer the “load” (the “load” in this case is the boat’s entire AC electrical system) between one of two AC power sources (either shore power or the onboard generator). The GTS must be constructed in a way that it simultaneously transfers the load’s “hot” lines (“L1 and L2”) and the load’s “neutrals” “N.” Figure 3 shows the electrical diagram of Sanctuary’s physical GTS. “Source 1” and “Source 2” are our 120V, 30A shore power inlets. “Source 3” is our 240V, 50A generator input (happens to be the way our generator is configured). In order to comply with the neutral-to-ground bonding requirements of the NEC and ABYC, the GTS is built to switch the neutrals as well as the “hot” lines. In this way, the required neutral-to-ground bond can be installed at the generator, aboard the boat, and the entire platform remains compliant with the ABYC electrical standard and compatible with the NEC for shore power.

About Shore Power Transformers

Shore power transformers are expensive, large, heavy and require significant physical space surrounded by free-flowing air for ventilation. These transformers can suppress spikes and electrical noise from entering the boat from the shore power grid. Some transformer designs can automatically compensate for “low” dock voltage (shore power “brownout,” normal 208VAC). There are two shore power transformer wiring configurations: an “isolation configuration” and a “polarization configuration.” In both cases, the transformer is installed aboard the boat. The secondary winding of the shore power transformer is defined to be the “derived source” of AC power aboard the boat.

For a 30A, 120V isolation transformer, the primary requires a double pole breaker, preferably fit with ELCI, which breaks both “L1” and the neutral, “N.” The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V branch circuits.

For a 50A, 240V isolation transformer, the “L1” and “L2” hot lines are brought aboard through a double pole disconnect breaker, preferably fit with ELCI. The pedestal neutral, N, is not brought aboard at all. The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V/240V branch circuits.

The difference between “isolation” and “polarization” is the wiring configuration of the safety ground. With isolation transformers, the safety ground in the shore power cord terminates at a shield in the transformer. With polarization transformers, the safety ground of the shore power cord is connected to the boat’s safety ground buss, and is brought back to the shore power pedestal. With a polarization transformer, it is best practice to also install a Galvanic Isolator in the safety ground wire.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for 125V/250V, 50A and 125V/250V, 100A applications  are manufactured in three “flavors:”

1. Basic, single input, single output, 240V transformer; least expensive flavor.
2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input for 240V output.
3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.  While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.  Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”  The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.   The output of this transformer (the secondary) produces single phase, 3-pole, 4-wire output which powers the boat.   In English, that means there is a conventional black, red, white and green output.   If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.  But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances. With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a “low” utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances. Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.   Microwaves will run but will take longer to cook.  Coffee pots will perk, but will take longer to do their thing.   Electric blankets will keep sleepers warm and cozy.  Water Heaters will heat water, but take longer to reach target temperature.   Stovetop burners will heat, but will take longer to get as hot. Heat pump compressors and fans should all run, but some “non-marinized” motors may overheat and cut out to protect themselves from damage; marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.  One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V “low voltage” should be designed to detect the condition, put up a power warning fault light, and self-disconnect.   Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer 120V marine heat pumps do that.

About Generators

An AC generator is a mechanical machine consisting of a propulsion engine that drives an alternator. The machine must spin at a constant rotational speed to maintain the 60Hz output frequency. The waveform from a rotating genset is a pure sine wave. Although gensets are rarely actually run at their full load capacity, AC gensets must be rated for the largest electrical load they will ever have to support. Mechanical speed controls in these machines add to the requirement for a relatively great deal of preventive and corrective maintenance. Replacement parts are expensive and heavy. An AC generator can power all normal household appliances including heat pumps. Considering capital expense and lifetime fuel and maintenance costs, AC gensets are inherently expensive, per kW-h, to produce AC electricity on a boat.

A DC generator can be a practical alternative to an AC genset for most cruising boats.    DC gensets such as those made by Alten®, Hamilton-Ferris®, PolarPower® and ZRD® are essentially used aboard as “motor-driven battery chargers.”  The AC power used aboard the boat arises from the battery bank via inverter(s).  Multiple smaller inverters can provide for staged comfort and convenience options as well as systems redundancy.   Because batteries can supplement total power demand (Kirchhoff’s Laws), DC gensets do not have to be rated for max demand, as do AC gensets.  When onboard loads are light, the DC genset provides enough energy to power both the inverter(s) (for conversion to AC) and the battery bank (for battery charging).  When demand exceeds the generator output capacity, the batteries themselves make up the difference.  This means DC gensets can be of smaller capacity and can adjust to light loads more efficiently than AC gensets. Since DC gensets charge batteries, they do not need to spin at a regulated speed and are mechanically less complex.  AC generators are sensitive to rotational speed to keep the AC output frequency at 60Hz, +/- two Hz.  The DC machine has no such restriction, and so are much more fuel efficient. Boaters faced with installing a net new genset or replacing an old genset would do well to consider the DC genset option.

High Complexity Aboard Boats – Inverter

From the perspective of “electrical standards,” boats are a sub-class of a larger category of “mobile platforms.” Inverter and inverter-charger devices can be installed in many types of mobile platforms, including cars, trucks, ambulances, emergency vehicles, RVs and airplanes. All classes of “mobile platform” have identical shore power interface compatibility requirements, and very similar user safety requirements. Inverters installed in host AC systems on boats carry significant complexity.

About Inverter-Chargers

An “inverter-charger” is an electronic device that converts DC from batteries into 120V/240V, 50Hz/60Hz AC and ALSO uses 120V/240VAC, when available from external sources, to re-charge battery banks. The shape of the AC waveform from inverters can be a “modified sine wave” (MSW) or a “pure sine wave” (PSW). PSW devices dominate in the marketplace in 2019, and since some electronic appliances do not tolerate MSW well, are to be preferred aboard boats.

There are two installation use cases that apply to any discussion of inverter or inverter-charger installations on boats.

Use case one:  consists of a stand-alone inverter that powers dedicated AC utility outlets that are separate and apart from the wiring and outlets of the host boat’s main AC electrical system. To have AC power at those outlets, the inverter must be turned “on.” When the Inverter is turned “off,” AC power is “off.” The AC wiring attached to this inverter would be expected to comply to the normal requirements for all AC wiring aboard. There is no automatic power transfer switching. Ideally, an inverter used in this way would feed a distribution panel that would provide overload protection to branch circuit wiring. The manual nature of this use case is not considered “desirable” by boat designers and builders. Specific standards for this use case are not enumerated in the ABYC E-11 standard, AC and DC Electrical Systems on Boats.

Use case two: an inverter or inverter/charger that is fully integrated into, and functions as a part of, the host boat’s AC electrical system.   There are no separate or isolated utility outlets. All powered utility outlets are overload-protected by the host system’s branch circuit distribution panel. Branch circuit utility outlets and appliances either 1) receive externally-provided AC power “passed through” the inverter or 2) receive AC power from the inverter via the energy stored in the boat’s batteries.   The inverter senses loss of external power automatically, and switching from external power to battery power is likewise automatic. User safety and convenience is maximized. This use case is covered in detail by ABYC E-11, AC and DC Electrical Systems on Boats, and ABYC A-31, Battery Chargers and Inverters. ABYC specifies device compliance with UL458 to maintain compatibility with neutral-to-ground switching aboard the boat.
As shown in Figure 4, when either shore power or generator power is available, the inverter automatically switches to “standby mode.” In “standby mode,” the internal transfer relay is energized by the external power source. The internal transfer relay has two functions. One is to pass the external power through the inverter (“passthru”) to the boat’s power distribution panel (red arrow), and the other is to simultaneously remove the device’s internal neutral-to-ground bond (red oval). This second function maintains compliance with the ABYC neutral-to-ground bonding requirements for shore power.Later, when external power is no longer present, the device automatically switches from “standby mode” to “invert mode.” As shown in Figure 5, the internal transfer relay drops, and the inverter begins to generate AC power by drawing energy from the boat’s batteries (red arrow). When the transfer relay drops, it simultaneously establishes the required neutral-to-ground bond (red oval). Since the inverter in “invert mode” is now the “derived source” of AC power, grounding the neutral via the internal relay maintains compliance with the ABYC electrical standard, E-11.

Inverters – Installation Impacts

Referring again to Figure 1, the “energy flow diagram” for Sanctuary, it is apparent that the 120V feed of branch circuits 1 – 3 and 4 are powered from either shore power or generator power through the Generator Transfer Switch. When either shore power or generator power is present, the inverter operates in “standby mode,” and AC for branch circuits 5 – 8 “passes through” the power transfer relay of the inverter-charger to feed AC from the respective source to those circuits. When the boat is under way, and an external AC power source is not present, the inverter switches to “invert mode.” In that case, branch circuits 5 – 8 are powered by the inverter-charger.

What is not obvious in the energy flow diagram is that, because the “hot” lines for circuits 5 – 8 originate at the inverter, the neutrals for circuits 5-8 must be separated from the neutrals of circuits 1-4. This is a manufacturer’s installation requirement for the inverter-charger device which has its origins in ABYC Standard, A-31, Battery Chargers and Inverters.

Inverters – Advanced Feature(s)

In 2019 in worldwide boating markets, Victron Energy B.V.® manufactures a series of inverter-chargers carrying the MultiPlus™ and Quattro™ brand names that have an advanced feature Victron® calls “Power Assist.” With this feature, the inverter is capable of “piggybacking” on top of a limited shore power source to boost the total amount of power available to power loads aboard the boat. Batteries are charged during periods of low demand, and support the inverter during periods of higher demand. Across a day of use, users must monitor the system to assure batteries are adequately charged.

A typical “Power Assist” scenario: assume a boat fit with one of these inverters visits a private residential dock, a public wall, or any similar location where only very limited AC shore power is available from a single 125V, 15A/20A residential outlet. Generally, 15A is not sufficient for powering boat loads by itself. That said, if the demand on the 15A circuit can be held below a level that causes the shore power overload circuit breaker to trip, convenience aboard the boat can be enhanced by the “Power Assist” feature. To ensure the inverter does not trip the shore power circuit breaker, assume the inverter’s shore power “Maximum Current” setting is 12A. As long as the “passthru” loads on the boat are less than 12A, the power for those loads comes entirely from the shore power outlet.  It is during these periods of light AC loads aboard the boat that house batteries are charged.

Later there may come a time that the load on the boat jumps up, perhaps because of a toaster, coffee pot, microwave or hair dryer. Assume that at some point the total AC load aboard the boat rises to – pick a number – 22A. Since the inverter-charger is limited to drawing 12A from the shore power outlet, the inverter itself jumps in to “assist” the shore power source with energy drawn from the boat’s batteries. The inverter will sync with the shore power sine wave, and 10A will be provided from the batteries by the inverter. Keep in mind that the inverter is designed to provide this assistance automatically, by monitoring passthru load and automatically jumping in to supplement loads that exceed the pre-set.

Functionally, the above is how the Victron® Power Assist feature works, and it has much user convenience appeal to boaters. However, there may also be an operational downside with the “Power Assist” feature. When this equipment attaches to the Electric Power Grid, it synchronizes it’s 60Hz power waveform with the power on the grid. Emerging experience suggests the synchronization process can cause out-of-phase currents that may trip dockside ground fault sensors.  Owners of these devices should be alert to nuisance trips when connecting to docks with ground fault sensors on pedestals.

Inverters without the “Power Assist” feature have an obvious “one-way” relationship with the Electric Power Grid; that is, they are loads that take power from the grid. Inverters with the “Power Assist” feature are electrically paralleled to the incoming shore power connection and can have a two-way interface with the incoming AC power grid. These two-way inverters are capable of delivering AC power backwards into the electric power grid to which they are attached. The ability to feed power backwards into the grid carries significant safety implications in certain fault scenarios.

“Distributed Energy Resources” (DERs) are AC electricity generating units, typically in the range of 3 kW to 50 mW, that are deployed across the power grid. DERs are installed close to loads, often on customer premises, often on the load side of the customer’s electric meter. DERs are designed to alternately draw power from and return power to the upstream hosting electrical power grid. Worldwide, DERs are a central concept to distributed solar and wind farm (“green energy”) production and to pumped-storage reservoir systems. DER technologies include 25kW to 500kW micro-turbines, 25kW to 250mW combustion turbines, 5kW to 7mW internal combustion engines, 1kW to 25kW Stirling engines, fuel cells, battery-based UPS systems, photovoltaic systems, and wind generation systems.

In the US, the NEC, state Public Utility Commissions, code enforcement Authorities Having Jurisdiction (AHJ), and the ABYC, have all recognized the safety implications related to DERs. While it would be rare – in 2019 – for power generated on a boat to be fed back into the local electric power grid, with a DER-capable inverter, it is possible. The “Power Assist” capability enhances living convenience for boaters as it does for land-based DER users, so its likely that inverter-type DER devices for applications aboard boats will only increase in availability in the future.

ABYC A-32, July 2017, is the most current electrical standard that governs the two-way interface of DER equipment when installed on boats. ABYC, A-32, AC Power Conversion Equipment and Systems, Diagram 1, is shown below. This diagram is the electrical “model” the ABYC has adopted for inverter-type DERs installed on boats. Referring to this diagram, the earlier discussion of neutral-to-ground bonding still applies. The relay that accomplishes that is shown in the green oval.

Inverter Safety – “Anti-Islanding”

In residential neighborhoods (and aboard boats), power arises from the local Electric Power Utility. If power is lost, the implication is that some part of the utility power grid failed. Causes can include electrical device failure, severe weather, floods, terrorism or severe mechanical insult (tree-fall on wires, vehicle into utility pole, hot air balloon into wires, etc). A loss-of-power event leaves some local geography without electricity; home(s), police/fire station, shopping center, hospital, farm, airport, etc., an entire neighborhood, a entire town, etc. Many affected entities have mission-critical needs for uninterrupted power, and use DERs to achieve that goal. The footprint of the area of lost power is referred to as an “Island;” that is, an area that is physically cut-off and isolated from the power grid.

For the safety of residents, rescue personnel and repair personnel working to restore power within the “island” of disruption, DER’s operating at the time of a power failure must immediately detect the loss of grid power and disconnect themselves to prevent back-feeding power into the “island.” Again referring to the ABYC diagram, the relay shown in the red oval is the means by which DER Inverters disconnect themselves from the grid. ABYC requires that the disconnect occur within 100mS of the loss of power. Note: the inverter may continue powering some or all of its attached loads, within the rated capacity of the inverter and the capability of the battery bank.

Boaters are NOT expected to understand or care how all this happens.  The net here is, boaters need to buy and install MARINE-CERTIFIED equipment for installation aboard their boats. Equipment from discounters like Harbor Freight does not meet these complex safety requirements.

Behind the words “MARINE-CERTIFIED” is a very complex series of electrical standards that spans the worldwide membership of the IEC. These standards define the mutually-cooperative manner in which DERs must interact with National Electric Power Grids. At the end of this article is “Addendum 1” that describes the safety and testing standards involved with DER equipment for those interested.

About Motors – Single-Phase

Single phase motors are more complicated than three-phase motors. Even small sized single-phase motors are more complicated – electrically and mechanically – than three-phase motors. The reason is that it is much more difficult to create a rotating magnetic field with just one, single-phase. The “natural” rotation of the phases of a three-phase machine does not exist in a single-phase machine.

There are several different techniques used to create a rotating magnetic field in a single phase motor. All of these motors have high inrush “surge” currents.

A shaded-pole induction motor is a relatively simple and inexpensive motor. There are no brushes. Starting torque is low, so these motors are used for fan and blower motors and other low-starting torque applications. Creation of the torque to start rotation is done by means of one or two turns of heavy copper wire around one corner of the field coil. When the field is energized, inrush current is induced in this heavy coil. This induced current is out-of-phase with the power line current. This results in a second, offset, magnetic field, which is enough to start motor rotation. These motors are generally made in fractional-horsepower sizes.

Where medium and medium-high starting torques are required, the split-phase induction motor is more appropriate. These motors also do not have brushes. Split-phase induction motors are built with two field windings. One of the windings is called the “start” winding and the other is called the “run” winding. One of the windings is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current is commonly created by feeding the winding through a capacitor. A common variation of this design is a switch that disconnects the capacitor when the motor is up to operational speed. In this design, a centrifugal switch is internally mounted to the armature. The switch opens to disconnect the start capacitor when the rotor reaches operating speeds. Often in motors of this type, there is an audible click of the centrifugal switch transfer as it opens and closes. This is normal. In compressor applications, another variation is to have capacitors in both the start coil circuit and the run coil circuit. These alternatives involve complexity and cost.

In addition to a start/run capacitor, another way to achieve a rotating magnetic field is with a second field winding with significantly different values of inductance from the main winding.  This effectively results in an out-of-phase current in the second winding.

Where small physical size and high torques are needed, the Universal Motor is preferred. Universal motors are expensive to build and require periodic maintenance. These motors have carbon brushes and complex internal components that create a strong, consistent magnetic field at all rotational speeds. They can start to rotate against high stall loads. These are commonly used in handheld tools (drills, saws, etc.) and kitchen appliances like mixers and blenders. These motors often are not rated for continuous use, because they generate significant internal heat in operation.

About Motors – Three-Phase

Three-phase motors are very simple electrical machines. Recall that in a generator, there was a rotating magnetic field inside three fixed armature spaced at intervals of 120°. Three-phase motors have field coils that are physically mounted at intervals of 120°. The incoming three-phase power is connected to the windings of the motor’s field coils. As the voltage in the phases rises and falls, each in turn, in the 60Hz sinusoidal rhythm, a magnetic field strengthens and weakens around the field coils. An aggregate rotating magnetic field is produced by the rise and fall of current in the three individual field coils. That aggregate magnetic field rotates around the diameter of the machine’s field coils at a rate of 60 times per second. Reversing the connections of any two of the incoming three phases will reverse the direction of rotation of the magnetic field, and therefore, the direction of rotation of the motor itself.

A characteristic of motors is that they have high start-surge currents. At the moment when power is first applied to the machine, this surge is at its greatest. As the motor spins up to its running speed, the current settles down to its steady-state running level. Motors have separate ratings for start and run currents. Circuit designers need to allow for start-surge currents in selecting the gauge of wiring to the motor. Large horsepower motors have special controllers that limit inrush surge, but small frame motors found in boats generally do not need these sophisticated controllers. Because of the inrush surge, motor circuits are generally set up with slow-blow circuit protection.

The strength of the magnetic field determines the amount of torque the motor can deliver. The work will be to turn pumps, fans, windshield wipers, machine tools, refrigeration compressors, etc. Starting torque is large because of large start-surge currents. Running torque is the steady state torque the motor produces. Engineers select motors to match the torque required by the machinery the motor will drive.

About Motors – Raw Water Pumps

In motor-driven water pumps used in terrestrial applications – a residential hydronic heating systems, for example – an electric motor connects to the pump via a mechanical shaft. A rubber “lip seal” is used in the pump housing to prevent leaks at the shaft. This design has it’s limitations. Over time, the lip seal will harden, crack and fail and/or the shaft will become scored from mechanical wear, leading to leaks. Obviously, this design represents a future maintenance activity for the owner.

Boat raw water pumps are of different design. Instead of a mechanical shaft, the motor is fit to a strong permanent magnet. The pump impeller is also magnetic, and rotates on a shaft mounted inside a Fiberglass Reinforced Plastic (FRP) housing. The pump housing is designed so that when fit together with the motor, the magnet fits inside the metal-containing impeller’s housing. Since there is no shaft penetration through the housing, there is nothing to leak. As the motor spins, the magnetic field acts through the FRP housing and causes the impeller to spin. Good installation practice is for the assembled motor and pump be mounted vertically with the motor above the pump.

This design is leak free. The impeller can jam, but the pump motor will not overheat and will not be damaged if it does. These motors generally need little maintenance, but check the manufacturers instructions to verify the needs of your pump motor.

About Motors – Maintenance

Routine maintenance for electric motors includes, first and foremost, periodic lubrication of sleeve bearings. Use machine oil, not automotive motor oil. Most motors have lubricating ports – small holes – for applying machine oil. Use only a couple of drops of oil. Avoid the temptation to flood the bearing. If you do, the motor will just throw the excess all over the place.

If a “capacitor start” or a “capacitor start/capacitor run” motor will not start, check the capacitor. When a capacitor fails, the motor may overheat and either will not start or will not run correctly. Capacitors are physically located outside the frame of the motor, and are much less expensive to replace than the motor. This is particularly true if the motor is an air conditioning/heat pump compressor sealed into a refrigerant system.

Brushes wear in normal service and are normal maintenance parts. Replacements are available from the tool or appliance manufacturer. Typically, the motor will show symptoms of impending failure. Brushes wear in operation to the point where they no longer make good electrical contact. Often, a small external physical bump will cause the motor to start. That’s a sure sign that the brushes need replacing. Order replacement brushes when symptoms first appear, or the tool will surely fail when you most need it, before replacement brushes are on-hand.

Motors are very reliable devices. Motors will generally give many years of satisfactory performance. The down side of that is that your specific model may not be available when you do need to replace it. If a motor will not start due to internal failure, you do have options. I recently had occasion to help a friend with a blower motor for his onboard air conditioning unit. The manufacturer wanted over $400 for a replacement blower. Instead, we took the motor to a local motor refurbisher, and for $60, the refurbisher replaced the bearings and rebuilt the motor. Centrifugal switches are also replaceable. Electric motor refurbishers are available in most medium sized and larger communities across the country. Don’t overlook this option. Look under “Electric Motor – Repair” in the Yellow Pages! Yes, folks, I grew up using Yellow Pages.

Refrigeration compressors have built-in safety circuits. One is a thermally operated switch that’s mounted to the case of the compressor. It is designed to open and disconnect power to the compressor motor if the compressor case gets too hot. Another is a pressure operated switch that is designed to disconnect the motor if the refrigerant gas pressure gets too high. Some units can also detect low refrigerant pressures. These switches can fail, and their failure rate is higher than the failure rate of the compressor itself. If a compressor fails to run, check the safety switches before changing the compressor or changing the entire fridge or air conditioner/heat pump unit. Many an unsuspecting soul has paid to have a compressor replaced and only gotten a $20 switch for the money!

Qualifications of Personnel

The above discussions illustrate an important safety consideration which I know some will find restrictive and controversial. Simply stated, people who are not thoroughly familiar with marine electrical standards and requirements should not install or modify boat electrical systems! Many excellent residential electricians and many skilled DIY “practitioners” who learned terrestrial NEC compliance techniques in residential applications are simply not qualified to perform work on boats. Switching requirements are different on boats than they are on land, yet it is true that cheaper switches incorrectly selected for use on a boat may appear to work. The details of neutral-to-ground bonding are much more extensive on boats, yet man-made wiring errors may go hidden and without symptom for weeks, months or years. Work performed by one who is simply unaware of boat equipment requirements can lead to unintended but serious safety faults for friends and family to discover at some random future time.

The frustration of encountering a no-power situation because the boat trips a ground fault sensing pedestal breaker on a cruise is unwelcome for the boat owner, but is truly unsettling to the spouse and guests aboard. Diagnosing man-made wiring errors is expensive and frustrating by any definition. It is extremely important to know, understand and comply with the low-level details of the ABYC electrical standards. Boats in marinas are in very close proximity to their dock neighbors. All marina residents – whether longterm or transient – depend on the safety of neighboring boats. When hiring someone to do electrical work on your boat, make sure the person you hire is actually qualified by training and certification to perform marine installation, maintenance, troubleshooting and repair services.

Incidental Topic – Dockside Ground Fault Sensors

While not actually a boat-side AC electrical topic, GFIs on docks is a topic that does apply to any discussion of boat AC electrical systems. The problems that cause dockside ground fault sensors to trip are all caused by conditions that exist on the boat. Many (the great majority) of these issues were caused by unqualified but well-intended DIY practitioners who did the wrong things without realizing it. I have written in detail about dockside GFI problems and solutions. Articles on this website that discuss these issues include:

  1. Electric Shock Drowning
  2. Emerging AC Electrical Concern
  3. AC Safety Tests for Boats
  4. ELCI Primer
  5. Ground Faults and Ground Fault Sensors
  6. Ground Faults: Difficult to Hire Skilled Troubleshooter

Incidental Topic – Galvanic Corrosion

Also not an AC electrical topic, this heading is included because the Galvanic Isolator
is fit into the main safety ground conductor of the boat. The submerged metal parts of boats are comprised of a mix of dissimilar types of metals. Boats commonly have
stainless steel drive shafts and rudders, bronze propellers, struts, rudders and thruhulls, and Aluminum trim-tabs. When immersed in sea water, these different metals and metal alloys follow the same laws of electrochemistry as found in a battery, albeit not optimized in construction and materials purity as they would be in a made-for-purpose battery. The action of this electrochemistry results in “metal wasting” corrosion of some of the underwater metals.

Another very common form of galvanic corrosion is “single-metal” corrosion (ex: “rust”
in iron-containing metals, “poultice corrosion” in aluminum, “pitting corrosion” and
“Crevice corrosion” in Stainless Steels). A serious and often unrecognized form of
single-metal corrosion occurs in the all too common brass plumbing fittings bought in
big box and hardware stores, and even in some marine chandleries. Brass is a metal
alloy containing primarily copper and zinc. We know zinc is a galvanically active metal
(anodic) that will sacrifice itself to protect more noble metals (cathodic). Brass fittings
flooded in sea water suffer from a phenomena called “Dezincification.” The zinc
wastes away, leaving the remaining metal structure of the brass alloy porous, with a
pink appearance, and physically very weak.  WARNING: never use brass fittings
below the waterline or in raw water circuits used by heat pumps aboard the boat.
“Sacrificial anodes” of zinc, aluminum and magnesium are usually attached to valuable underwater metals to protect the more valuable metals from galvanic corrosion wasting damage. Zincs are most effective if electrically located on the metals they protect.  Zincs waste away as they give up positive ions to the electrolyte of the galvanic cell.An “Impressed Current Cathodic Protection” (ICCP) device is an electronic approach to
managing galvanic corrosion on boats built with metal hulls (steel, aluminum). An
ICCP is able to protect the relatively very larger surface areas of metal hulls than can
be done effectively with individual sacrificial anodes.

About – Galvanic Isolation

The ABYC recommends some form of galvanic corrosion control on boats. Aside from
the active electronics of an ICCP, there are three passive ways to achieve this control.
One modifies the electrical makeup of the underwater collection of metals. The other
two act by disrupting the flow of the small but destructive DC galvanic currents. The
latter two approaches impact upon the design of the boat’s shore power safety ground.

The first and most common approach to reduce galvanic wasting is with the use of
sacrificial anodes. These anodes modify the makeup of the underwater metals in a
way that makes them waste, rather than more valuable metals.

The second approach is with the use if a Galvanic Isolator (GI), which eliminates the
electrical path for galvanic currents to use. Electrically, this device is placed in the
main safety ground wire where the ground conductor enters/exits the boat; that is,
electrically at the shore power inlet(s). The newest generation of GI is the “Fail Safe”
device. It consists of a solid state, full wave, bridge rectifier and a large capacitor. This device will allow AC fault currents to flow normally in the safety ground, should
that need ever arise. The physics of the diode junction effectively blocks the small DC
voltage that drives the flow of galvanic currents. Without a galvanic isolator, zincs can
be consumed in weeks. With a galvanic isolator, zincs should last many months.

The third approach to interrupting the flow of galvanic currents is by installing an
onboard AC “shore power transformer.” An Isolation configuration eliminates the path
from the boat’s grounding network to shore. A polarization configuration keeps the
shore path, so should include a Galvanic Isolator. There are subtle pros and cons to
this choice. This author prefers the polarization configuration for maximum safety.

Electrical Emergencies

True electrical emergencies are rare. Electrical emergency situations will always
become less dangerous if power is quickly disconnected.

Be wary and suspicious of unfamiliar, unpleasant or pungent odors. Transformers,
motors and many other electrical devices that are in the process of failing often
overheat and cause insulating materials to emit strong, pungent odors. TURN OFF
POWER and use your nose to track down the source. Turning power off will also
shut down air circulating blowers that circulate odors and make locating their origin
difficult. Treat strong odors as an pre-emergent true emergency. The goal is to
find the offending device before it ignites! Turning off the power will stop the self destructive process and allow the failing device to cool off. Do not re-start a device
that has overheated in operation to the point of emitting strong odors! This type of
over-heating often causes secondary internal damage that you cannot see.

In an emergency, the most important commodity you can have is time! Time to
think and act. To buy time, install smoke detectors. Install smoke detectors that
have dual mode incipient gas sensors as well as visible smoke sensors. Install a
model that communicates with other units so that when one alarms, they all alarm. I
placed a dual-mode smoke detector on the overhead of my electrical locker. That
locker is a small, closed space behind my AC and DC branch circuit panels, and is
where the shore power inlets and the Generator Transfer Switch are located. That is a
good place to install a smoke detector, placed there in order to buy me some time.

Emergencies – Avoidance

When working around electricity, use insulated tools, especially when working around batteries. Batteries contain enormous amounts of stored energy. A metal tool across the terminals of a battery may actually weld the tool metal to the battery terminals. If this happens, the tool metal will become extremely hot. Whenever you plan to work around a battery, pre-plan to have a two foot piece of 2”x2” wood stock, or a wood handled carpenter’s hammer, readily at hand. If the worst should happen, use the wooden 2”x4” as a mallet to forcefully knock the tool away from the battery terminals. Once this cascade of events starts, the only way to stop it is to break the tool free of the battery terminals. Act quickly. The battery can get hot enough to melt and start a fire.

Many electrical emergencies are avoidable. Always comply with standard electrical
safety rules and practices. This is not a exhaustive list. As you plan your projects,
plan for safety.

    1. Never work on live electrical circuits. Turn power “off” before accessing.
    2. Never work alone; always have someone with you who can disconnect power
      and call for help in an emergency.
    3. Never wear watches or jewelry when performing electrical work.
    4. Never parallel multiple small gauge wires to achieve a larger current carrying
      capacity (“ampacity”). That virtually guarantees trouble in the future.
    5. Install protective insulation and safety covers to prevent accidental contact with
      bare electrical connections and terminals.
    6. Periodically, go on an “inspection tour” of the boat’s electrical system; make this
      a part of your scheduled preventive maintenance checklist. Specifically,
  • Screws work loose over time; with power off, periodically go through the boat
    and tighten electrical connections.
  • Crimp connections corrode and loosen over time; avoid crimp connections
    wherever possible; given the choice to splice an existing wire or run a new
    wire, run the new wire; with power off, check crimps by firmly pulling on the
    wire at the crimp. Replace any connections that show any signs of heating
    or of being or becoming loose!
  • Secure loose or dangling wires.
  • Check wiring bundles where they ride over or round obstructions or through
    bulkheads. Vibration injures insulation and wiring, so support and insulate
    bundles in these areas to prevent wear spots.
  • Leave adequate slack in wire runs so they are not under tension.
  • Repair cuts, cracks or gouges in insulation immediately. Don’t wait.

In Case Of Fire

“Experts” all agree, in any fire on a boat, 1) there is very little time to act, and 2)
the odds of successfully fighting a fire are against you from the beginning.

If there is any doubt about being successful at extinguishing a fire aboard, use the
precious little time available to get your crew and yourself safely away from the fire.

    1. Alert your crew:
      • If you decide to fight a fire, do not use water! Water can conduct electricity,
        and you may wind up with both fire and electrocution emergencies. To fight
        an electrical fire, use a dry-chemical extinguisher rated for “Type ABC” fires.
      • Crew calls “m’aidez” (“May Day”) via VHF-16, or 911 via telephone. Do not
        hang up the phone until the 911 dispatcher tells you to.
    2. Disconnect Power:
      • If on shore power, turn power “off” at the pedestal!
      • If on genset, shut down the machine!
      • Shut down DC power to any inverter or inverter/charger!
      • Disconnect the main battery bank!
    3. Once the fire is extinguished, monitor the involved area to be sure it’s cool
      enough that it will not self re-ignite.
    4. Make repairs before re-applying power.

Appendix 1

The following is more in depth than I usually write, and will be of interest to advanced
DIY practitioners and electrical professionals interested in how electrical safety and
testing codes are applied. This material adds to what has been presented above, but
is not necessary to understanding.

Acronyms and Abbreviations

ANSI – American National Standards Institute
AHJ – Authority Having Jurisdiction
CSA – Canadian Standards Association
DER – Distributed Energy Resources
DOE – United States Department of Energy
EPS – Electric Power System
ETL – Intertek® registered testing mark (Electrical Testing Laboratories)
IEC – International Electrotechnical Commission
IEEE – Institute of Electrical and Electronics Engineers
NEC – National Electric Code (United States)
NREL – National Renewable Energy Laboratory
PUC – Public Utility Commission
REPC – Rural Electric Power Conference
SGIRM – Smart Grid Interoperability Reference Model
UL/ULc – Underwriters Laboratories® testing mark

ABYC A-32, AC Power Conversion Equipment and Systems

All ABYC Standards follow a common layout format (“boilerplate”). Following is an
excerpt from the “References” section of ABYC A-32, July, 2017:
32.3 – References
The following references form a part of this standard. Unless otherwise noted the
latest version of the referenced standard shall apply.
32.3.1 – refers to several other ABYC standards
32.3.2 – IEC 62116 Test procedure of islanding protection measures for utility interconnected photovoltiac inverters (IEC 62116 is a European standard)
32.3.3 – IEEE 1547 Standard for Interconnecting Distributed Resources with the
Electric Power Grid (IEEE 1547 is a US ANSI Standard (North America power grid))

Author’s note: emphasis and comments added for clarification.

Relationship of IEEE 1547 and UL 1741

Safety Standards define minimum feature and function capabilities for the design of a
particular class of equipment; in this case an inverter-charger DER. Testing Standards
define test specifications that a device must meet in order for the manufacturer to claim
compliance to the design standard. This leads to some very complex relationships
between different national regulatory authorities and between and among multiple
independent, private enterprise businesses. Following is a pictorial that shows the
relationship of the safety and testing standards that define the INTERFACE between
devices in the class of DERs to the North American Electric Power Grid as deployed in
the United States:

In the above Figure 6, the IEEE 1547 Standard defines the minimum design
requirements of DER equipment. IEEE 1547.1 and UL 1741 together define the
minimum test conditions that the completed device must meet. In addition, ABYC A32,
32.9.2 calls for disconnect protection in less than 100 mS after loss of incoming AC
power. The NEC, Article 705, defines what the National Electric Power Grid is
expecting.

Compliance

Figure 7 shows the Certificate of Conformity for the Victron® MultiPlus™ device family.  At the bottom of the Certificate, readers can see that the device complies to UL 1741-2016 (2nd Edition) and the Canadian National Standard, CAN/CSA 22.2, No. 107.1-16, (4th Edition).

Summary

  1. Victron® MultiPlus™ and Quattro™ inverter-chargers are grid-attached DERs,
    even though their purpose when installed on boats is not to supply power
    backwards onto the grid.
  2. ABYC Standard A-32 incorporates the requirements of IEEE1547. All of the
    ABYC standards operate in the same way, by including (incorporating) other
    relevant IEC, EN and IEEE standards into themselves.
  3. IEEE 1547 has been adopted as an American National Standard by the
    American National Standards Institute. IEEE1547 and subs (1547.1 through
    1547.8) state the design and testing requirements that DERs used in the US
    must meet; in this case, we are specifically interested in the Victron® MultiPlus™
    and Quattro™ inverter/charger devices. Victron® complies to UL1741, which is
    compatible with the NEC in the US.
  4. At this writing, I am still investigating, but I believe it is true that when UL 1741 applies to a device, that certification supersedes UL 458. UL458 compliant
    devices disconnect the incoming mains when the device is operating in “invert”
    mode. UL1741 compliant devices do the same thing, but for a broader set of
    considerations.
  5. Per Victron®*, “…we disconnect/get isolated from the AC source within 20mS.”
    That is well within the July 2017, ABYC A-32 requirement of 100mS.

* email to the author dated 3/15/2019, signed:

Mr. Justin Larrabee
Sales Manager
Victron Energy
70 Water Street
Thomaston, ME 04861

AC Electricity Fundamentals – Part 1

2/14/2019:  Initial post
6/6/2020:    Added: “US Utility Voltage Standards vs. Common Language”
10/6/2020:  Added section: “Electric Circuit “Wire” Naming and Identification“;
                      other typo and grammar edits
10/10/2020: Revised the sections “Ground” and “Common/Circuit Common.”

About this article

This article discusses the concepts and terminology of AC electricity at an introductory level. The scope of the article is limited to the AC power systems found in North and Central America. In Part 1 (this part; already plenty long enough), I will discuss the basics of AC power generation and the delivery of AC power to single-family residential neighborhoods and homes. In the Part 2 article,  I discuss the AC power systems as found on cruising boats.

I chose this two part approach for two reasons. First, almost all homeowners have some familiarity with household AC electricity. At the very least, most homeowners can find the circuit breaker panel and reset tripped breakers. Second, and more important, boat AC electrical systems are just a specific subset of what is found in a single-family residential AC installation. Boat AC systems are equivalent to sub-panels in a residence. Sub-panels are subordinate to the main service disconnect panel in a residential building, and in the same way, boats are subordinate to the AC electrical infrastructure of a marina. A basic understanding of household AC electrical systems puts boaters 75% of the way towards understanding boat AC electrical systems. Where boats differ from land-based residential buildings, the reasons are based on specific safety issues that emerge in, and are unique to, the marine environment. Boat AC electrical systems are significantly more complex than single family residences.

My goal is to explain electrical topics in a way that enables readers to relate new information to what is already somewhat familiar, so that boaters can identify and summarize problems or questions, and communicate with, interpret and understand the service professionals with whom they may need to interact.

Safety

There is one absolute, always rule whenever you must deal with electricity. VIRTUALLY ALL ELECTRICIY CAN BE DANGEROUS TO PROPERTY AND LIFE. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.  The large batteries found on boats can produce explosive gasses and store enough energy to easily start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! If you will be working in noisy environments, with running engines or other loud machinery, WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!

The rule is, “if you aren’t sure what to do and how to do it, stop. Don’t do anything until you’re sure of the “what,” “how” and “why!”

Electrocution

Electrocution is a biological insult that starts with an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death.  Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electrocution, there are several things to do immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM!  You can not save someone else if you panic!
  2. Avoid becoming a victim yourself!  DO NOT TOUCH THE VICTIM, METAL MACHINERY OR NEARBY METAL OBJECTS IF POWER IS STILL PRESENT! 
  3. SCREAM FOR HELP! ATTRACT ATTENTION!  Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  4. REMOVE THE POWER SOURCE FROM THE VICTIM BY DISCONNECTING THE ELECTRIC POWER at the pedestal.
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  6. THROW LIFE RING TO VICTIM.  DO NOT ENTER THE WATER YOURSELF!
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again!  Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate.  CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.
  10. CONTINUE CPR UNTIL THE VICTIM REVIVES, UNTIL EMS ARRIVES TO RELIEVE YOU, OR UNTIL YOU ARE PHYSICALLY UNABLE TO CONTINUE!

Basic Electrical Working Concepts  – Volts/Amps/Ohms

Like gravity, electricity is invisible. A common analogy used to explain electrical concepts is to liken an electric system to a community water system. Consider the familiar garden hose fit with a nozzle. In the garden hose, when the nozzle is opened, “pressure” in the system makes water flow.

In this analogy, the water in the hose is analogous to electrons in a wire. “Voltage” is the “propulsive energy,” or “pressure,” that makes electrons flow through the wire. The greater the water pressure, the more water flows per unit time. Similarly, the more voltage that is present across a circuit, the more electrons will flow through the circuit per unit time. The amount of water that comes out of the hose is measured in “gallons.” The flow of electrons through wire is measured in “amperes,” or “amps.”

In a water hose, the nozzle restricts the flow of water through the hose. The flow of electrons is restricted in electrical circuits by the electrical property of “resistance.” All materials that conduct electricity have some amount of resistance. Silver and gold have little resistance per unit length. Pure copper has only slightly more, and aluminum has slightly more again. Even the small resistance of a copper wire is extremely important in power distribution applications. Electrical “resistance” is measured in “Ohms.”

Assume we have a 3” diameter water hose and a 1/2” diameter water hose, both attached to the same water source. Only so many molecules of water can fit through the small hose in a minute, but many more molecules of water can fit through the large hose. This concept is called “carrying capacity.” Only so many electrons can “fit” through a wire per unit time.  The larger the wire, the more electrons.  Electrical “carrying capacity” is called “ampacity.” “Ampacity” is a rating assigned to wires.  Wires of the same metal, of different sizes and covered by insulation with different thermal and chemical properties, have different rated “ampacities.” The ampacity rating is the safe maximum current the wire can carry within the temperature rating of the wire’s insulation. Ampacity tables are widely available on the Internet.

Ohm’s Law – Memory Aid

The mathematical relationship between voltage, current, resistance and power is defined by “Ohm’s Law.” Ohm’s Law is probably the most fundamental relationship there is in the entire realm of electricity.  Folks who deal with electricity regularly have this relationship emblazoned in their brains, but for the rest of us, this “memory aid” is  extremely helpful! First, decide what variable you want to calculate. It’s unusual not to know at least two of the necessary variables. For example, today I saw a TV advertisement for a small, portable, plug-in electric space heater. The device plugs into a 120V outlet, so we know E = 120V. I went to the website and found that the unit is rated at 600 Watts, so we know P = 600. For use on the boat, I wondered how much current the device would draw. From the two known variables, we can calculate that the unit will draw about 5 Amps of AC current, which indeed may be OK for some uses on a boat. We also know the unit’s equivalent resistance is 24 Ω (“Ω” is the Greek Letter “Omega,” and is used as shorthand for “Ohms.”)

Ground

The word “Ground” has multiple meanings in different contexts.  To a residential electrician, “ground” should mean the “safety ground” required by the National Electric Code (NEC).  But in casual conversation around docks, “ground” may mean lots of things.  In DC discussions, “ground” probably refers to a “return conductor” for DC electric current.  But in AC discussions, the “return path” for electric current is the “Neutral.”   So, what does the word “Ground” really mean?  Unfortunately, it means all of the above, and more.

The Earth – the crust of our beloved home planet – is electrically conductive. It has many minerals and mineral salts which provide “free electrons.” In the presence of an electric voltage, electrons flow from point-to-point around and within the earth’s crust. By far the most dramatic example of this is the natural phenomena called “lightening.”

The electrical potential of the earth is defined to be “zero” volts. It is the standard reference point for taking electrical measurements.  In the rest of the world, connections to the earth are referred to as “Earthing,” but in North America, we talk about connections to the earth as “grounding.”  In order to connect a residential electrical system to “earth ground,” one or more interconnected rods (usually of copper) are driven into the earth. The neutral return point of the residence’s electrical system is physically connected to the network of copper grounding rods.

The concept of “earth ground” is not absolutely essential for the safety of people, pets, farm animals and wildlife.  The entire electric distribution grid of the country is connected at innumerable points to rods driven into the earth (the “electric grid” is a “multi-earthed system”).  Every residential property has an “earthing” connection at the service entrance to the residence.  Earth ground is also the reference for rendering static and lightening safe.  In electrical system design, protecting manmade structures from lightening involves routing the discharge arc around the structure to be protected, not through it.

Under some unique and undesirable circumstances, electric currents can be found flowing through the earth.  When that happens, it can be dangerous – even fatal – to people, pets, farm animals and wildlife.  This is most likely to occur, and most serious, around high voltage power distribution lines and high voltage switchyard facilities.

The essential point here is that “earth ground” is a universally understood reference point for all power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the total absence of any voltage.  We will return to this concept over and over as we proceed in our discussion.

Circuit Common/“Common”

The concept of “earth ground” is essential for electrical system designers to manage weather catastrophes and other accidental insults to electrical systems, static and lightening, but an “earth ground” is not necessary for electric circuits to operate.  Most, if not the vast majority of, portable generators ARE NOT grounded, and yet household appliances and tools run by portable generators run just fine without that ground connection.  People may get hurt doing that, but the appliance and tools themselves are able to run just fine.

The term “common” is useful in electronics applications.  The term “neutral” is used in residential AC electrical systems to reference the conductor that returns current flowing in a circuit from the load to the source.  The “common” conductor does not have to be “0” volts with respect to ground. Unless the neutral is specifically referenced to ground, the “common return” is a “free-floating” conductor. It is extremely important to understand the difference between the concepts of “ground” and “common.”

Unfortunately, the term “common” or “circuit common” is not often used in routine conversation.   The common return of a circuit is frequently – colloquially – called its “ground,” to mean the return path for current.  “Neutral” is a specific term that refers to the current-carrying return conductor of residential AC circuits, and although in AC electrical systems, the “neutral” is connected to “ground,” the term “neutral” is not a specific reference to “earth ground.”

Direct physical contact with energized high voltage is completely safe as long as you are not “across” two or more electrical conductors. For current to flow, there must be a connection between two conductors where there is a voltage difference between them (that is, “across a voltage”). Consider birds sitting on high tension transmission lines, or squirrels running along neighborhood overhead wires. They are safe because they are on, but not across, a voltage. The animal’s entire little body is raised to the voltage of the wire upon which they sit, yet they are perfectly safe because there is no path for current to flow THROUGH the body. The electrical activity of their brains and hearts is not affected by the voltage they are sitting upon. But, a human-being on a wet concrete floor wearing leather shoes best not come into contact with a “hot” wire. That concrete floor is made with salt-containing minerals, and most definitely is electrically conductive, especially when wet. A person standing on that floor and simultaneously touching an energized wire is “across” an electric voltage. That is a shocking experience!  Maybe, a fatal, shocking experience.

“Conventions” vs. Facts

Within the study of electricity as a science, there are hard electrochemical and materials facts, and then there are shorthand ways people talk to each other about complex concepts.  This happens in all professions, of course.  It’s all fine until the terminology confuses an understanding of the true concepts.  Some examples:

  1. It is a fact of physics that electrons carry a negative electrical charge, which means electrons flow from a more negative voltage in a circuit to a more positive voltage.  However, by universal agreement, or “by convention,” the entire practice of electricity and electronics treats current as flowing from positive to negative.  The direction of electron flow has no practical importance, but to properly interpret electrical diagrams, you need to understand the conventional way current flow gets represented by arrow-containing symbols.
  2. The symbols on electrical drawings are all agreed by “convention,” or “working agreement.”  Industry-specific symbols are agreed by international standards organizations.  Where there are symbol differences, their meaning is often obvious.  Some differences occur across international boundaries.  The power industry uses different symbols than are used in the electronics industry.
  3. The “single phase, center tapped, three wire” service is the residential standard in use, by convention, all across North and Central America.  It is institutionalized in the National Electric Code of the US and The Canadian Electric Code in Canada.  Completely different systems are used in other parts of the world, including Europe, Asia, Oceania and southern South America.
  4. The insulation used to coat electrical conductors is colored.  The colors, by convention, identify the use to which wires are put.  Understanding the color schema for wires is essential to electrical safety.  Mistakes here can be fatal.  The meaning of colors vary from country to country.  There are numerous differences between the United States and the nations of the European Economic Community and Oceania.  For those interested, tables are available on the Internet that document color meanings.

Science and Craftsmanship

The laboratory study of “electrical energy” is a theoretical and conceptual science.
Electrical craftsmanship is practical.  I will discuss only a tiny subset of the technical terms and concepts that are necessary to understanding low voltage AC as found in residential and boat applications.  Craftsmanship involves selecting materials, employing fabrication techniques, installing and maintaining electrical equipment, all with the goal of accomplishing some intended design purpose.  Craftsmanship is performed by electricians or electrical technicians and governed by formal regulatory controls called “electrical building codes.”

I view craftsmanship in two stages, which can be sequential or iterative.  If you have ever done an electrical project, you’ve performed both of these functions.

The first stage is the domain of the “circuit designer;”  i.e., the person who designs a branch circuit for installing a ceiling fan with a single switch to turn the fan “on” and “off.”  Or a slightly more complex branch circuit with three switches to turn a light “on” and “off” from different locations.  Or a much more complex array of multiple branch circuits to power a “man cave” or “she shed.”  Or the system for an entire home.  The designer must have solid knowledge of the National Electrical Code (NEC).  Electrical designers for boating applications must be thoroughly familiar with the American Boat and Yacht Council (ABYC) electrical standards.  The NEC and ABYC standards have as their purpose avoiding or minimizing present and future loss of life or damages to property.  The work product of the designer is a system drawing that defines the purpose of a circuit and the manner in which that purpose will be achieved through the use of electrical equipment, components and materials.  The work product includes a the bill-of-materials of the components required to implement the project.  For most projects, a reliable cost estimate can be produced at this stage.

The second craftsmanship stage is the domain of the skilled technician who is charged with the doing of the thing.  This craftsman must know how to use and interpret the designer’s drawings and how to use an enormous array of electrical meters and mechanical tools in the safe fabrication, construction, installation and maintenance of electrical circuits.  This craftsman must understand current assembly techniques, materials and supplies, and must understand and deeply respect industry safety practices.  Safety practice involves knowing when to and when not to work around, and with, energized electrical circuits.  On boats, because of the special safety implications of an electrical system on a floating structure, this craftsman must understand not only what to do and how to do it, but in fact, why things are done as they are, in making an electrical installation safe.

Key Concepts and Terms

  1. Ohm’s law – describes the mathematical relationship between voltage, current, resistance and power.
  2. Kirchhoff’s Laws
    • Kirchhoffs Current Law: “the total current entering a node is exactly equal to the current leaving the node.
    • Kirchhoffs Voltage Law: in any closed loop network, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop.
  3. voltage – (Volt) the quantification of “Electromotive Force” (EMF) (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive Force is measured across two points in a circuit.
  4. current – (ampere; amp) a quantification of the number of electrons flowing through a circuit at any one time.
  5. resistance – (Ohm) a characteristic of an electrically conductive material that tends to retard or impede the flow of electrons through it.
  6. power – (elect: Watt; Joule) (mechanical: inch-pounds, foot-pounds) elect: the amount of “work” that electricity performs in its application.  In purely resistive applications, light or heat.  In turning a motor, torque.
  7. frequency – (Hertz) the number of times a wave goes through a complete cycle in a standard measurement time interval, usually one second.
  8. ampacity – (Amp) a rating of the ability of a conductor of given material, diameter and insulation properties to conduct an electric current within the temperature limit established by the properties of the wire’s insulation characteristics.  (current: amperes; amps) (temperature: degrees Centigrade)
  9. source – the origin from which AC power emerges to energize a circuit.
  10. load – the components of an electric circuit where energy is consumed to do useful work; “useful work” includes production of heat, light, or torque via a motor.
  11. common – a portion of a circuit connection or set of connections that creates a direct return path for electrons flowing in an electric circuit.
  12. neutral – a special case in an AC circuit of a non-ground return path for electrons flowing in a North American standard residential electrical service.
  13. ground – a universal  standard earth reference voltage of “0” volts.
  14. node – a junction, connection or terminal within a circuit were two or more circuit branches are connected together.
  15. fault current – an abnormal path for current flow, usually to ground.  Fault currents represent potentially dangerous conditions.
  16. short circuit – a specific category of electrical fault resulting from an unintentional direct connection of an energized conductor to either a return circuit or an equipment ground.  This low-resistance, unintentional connection results in the flow of extremely large fault currents, and causes overload protection devices (fuses, circuit breakers) to open in order to disconnect the energized power source.
  17. GFCI (Ground Fault Circuit Interrupter) – an anti-shock safety device that senses leakage currents and disconnects the energized power source.
  18. AFCI (Arc Fault Circuit Interrupter) – a fire protection safety device that senses lose connections and disconnects the energized power source.
  19. GFP/EPD/ELCI (Ground Fault Protection/Equipment Protective Device/Equipment Leakage Circuit Interrupter) – similar to GFCI, but higher disconnect specifications.
  20. chase, raceway, conduit, “emt” – enclosed containment spaces in a building or a boat through which wires are run to achieve access to distant locations or to protect wiring from accidental physical damage.
  21. Field Coil – the rotating part of one design of AC generator; this coil can be a DC permanent magnet (typical in small machines), or a DC electromagnet.
  22. Voltage Regulator – the device that determines the strength of the magnetic field in an AC genset by adjusting DC current flowing in the spinning field coil.
  23. Stator – the fixed coils of one design of AC generator, from which sine waves of AC power emerge.
  24. Armature – the power-producing component of a generator; the rotating part of a DC generator; the fixed coils (Stator) in one design of AC generator.
  25. switchgear – a generic term for all disconnecting devices (fuses, circuit breakers, switches, power panels).  This term is used across the electrical power industry, from generating station to transformer yards to residential locations.
  26. Inductance (Ohm)/capacitance (Farad)/Power Factor (unitless) – technical characteristics common to the behavior of AC electricity in circuits that significantly affect large motor driven appliances and all electronic devices.  These become increasingly important as voltages, frequencies and power consumption rise.
  27. Managing the collapse of a magnetic field – a design consideration of any magnetically operated electrical devices (motor, generator, relay, etc), and many solid state devices.  A significant safety consideration for maintenance craftsmen.  When a magnetic field collapses, it creates a very high energy spike, which sometimes includes an electric arc.
  28. ABYC – American Boat and Yacht Council, Annapolis, MD.  This organization produces a very comprehensive set of electrical standards applicable to boat manufacturers, the marine insurance industry and boat owners.
  29. NFPA – National Fire Protection Association; owner/creator of the NEC.
  30. NEC/CEC – National Electric Code (USA), Canada Electric Code (Canada).  electrical design standards for political subdivisions and the construction industry.  Ranges from codes for residential housing, light commercial and industrial buildings, elevators, hospitals, airports, and heavy industry.

Generation (Source) and Consumption (Load)

There are three primary divisions of all electrical power distribution systems, including the global system we call the “nationwide electrical power grid.”  They are 1) the source of the electrical power, 2) the transmission system, or interconnecting wires and switches that carry power from the source to the point where it is consumed, and 3) the load, or the part of the system where the electrical energy is transformed into useful work.

At the level of the US national electric power grid, the source of AC electric power is one or more generating machines located in one or more generating stations.  Often, the term “alternator” is used interchangeably with the term “generator.”  These generating stations range in size from enormous, industrial-sized installations to small rural hydroelectric dams to units suitable for individual residential applications.

The substations, switchgear and wiring that connects sources of power to load centers are extremely complex, involving may hundreds of miles of high tension power lines, enormous transformers and highly complex switches.  Transmission equipment  can also be as simple as an extension cord run from the garage to the hedge clipper.

Electrical loads fall into the entire range of electrical equipment, from the largest commercial synchronous motors to the smallest and most humble LED electric clock.

About AC Generators

Typical AC electric generators have a rotating magnet (imagine the big bar magnet you played with in grade school science) which has a north pole and a south pole.  That magnet may be driven by a belt, wind or water turbine or direct drive, but ultimately, it’s a spinning magnet mounted on a shaft.  The north and south poles of the spinning magnet travel in a circular path.  A pick-up coil is positioned just outside the edge of the circle.   As the magnet spins on it’s shaft, the poles of the magnet approach the fixed pick-up coil, producing an electric voltage at the pick-up coil.

As the spinning magnetic pole gets physically nearer to the pick-up coil, the voltage at the pick-up coil gets progressively larger.  Once the magnetic pole moves past and away from the pick-up coil, the voltage at the pick-up coil gets progressively smaller again. When the north and south poles of the magnet are both equally distant from the conductors of the pick-up coil, no voltage is produced at the pick-up coil.

The voltage induced in the pick-up coil by the passage of the north magnetic pole is equal in magnitude and opposite in polarity from the voltage induced by the passage of the south magnetic pole.  One pair of north and south magnetic poles that sequentially rotate past the pick-up coil produce one cycle of AC voltage at the pick-up coil on each revolution. The speed, in revolutions per minute (RPM), of the spinning magnet determines the frequency (Hz) of the generated voltage.  The resulting AC wave form is called a “sine wave,” which is centered around “0” volts.  Sine waves rise and fall in smooth, graceful fashion with no sharp transitions in the shape of the wave.

In the preceding diagram, there is a geometrically balanced arrangement of a spinning magnet and a geometrically balanced arrangement of pick-up coils.  The output power of the generator is directly proportional to the strength of the magnetic field, up to the limits of its materials and mechanical design.  The output consists of two wires, and is referred to as “Single Phase” AC.

Many physical arrangements of the magnet poles and pick-up coils are possible, but the basic principle is the same for all AC generators.  To produce 60Hz AC, a single phase, two-pole, gasoline-driven, big box store genset (2500W to 6kW) typically spins at 3600 rpm; a single phase four-pole Marine genset (7.5kW to 25kW) typically spins at 1800 rpm. Because of the enormous weight and mechanical forces involved, multi-megawatt commercial generators may have 24 poles and spin at 200 rpm.

The rotating magnet in an AC generator is called the “field coil.”  The field coil is just a spinning DC electromagnet.  DC is fed to the field coil via slip rings and brushes on the spinning shaft.  The fixed pick-up coil in an AC generator is called the “stator coil.”  It is wrapped around iron support columns that are fixed in position around the perimeter of the frame of the machine.

Occasionally, the term “armature” may be heard; an “armature” is defined as the power-producing component of a generator.  In a DC machine, it is the armature that spins, with field coils stationary in the frame of the machine.  Fixed field coils with a spinning armature is a construction alternative for small frame AC alternators (<25kW).  This is both more costly to build and much more complex mechanically, so not common in the generator sizes found in the consumer retail market.

The amount of power that a generator can produce depends on many aspects of the physical construction of the machine, the amount of energy available from the driving motive source, the size of the internal conductors and underlying metal components, the strength of the internal magnetic field, and many other factors.

“Single-Phase” and “Three-Phase Power”

In reading through questions and discussions on various Internet boating bulletin boards , the differences between “single phase” AC and “three-phase” AC is often a point of confusion.  Three-phase power is extremely rare in residential settings, and few people have any life experience with it.

Consider the above preceding description of generator concepts.  Commercial power plants are fit with enormously large and heavy generators.  For several reasons, it is advantageous for these very large machines to spin slowly.  These generators are built with a large number of physical pick-up coils.  These pick-up coils are arranged as pairs in sets of three.  Logically – not physically – the machine appears as shown in this diagram.  These sets of pick-up coils are placed around the perimeter of the circle of the rotating magnet, at geometric intervals of 120° around the 360° circle.

As the bar magnet spins, the voltage in each of the pick-up coils rises to its positive maximum, falls back to zero, then rises to its negative maximum, and falls back to zero. This happens in each set of coils, in turn.  The result is three sinusoidal waveforms being produced by the same rotating magnet (“field”).  The three wave forms are displaced in time by 1/3 of a cycle (120°) of rotation of the rotor.  Enter here, the short-cut language the electrical industry has for this: “three-phase AC,” often shown on electrical diagrams written as “3-ϕ” or “3-phase.”

For commercial power plant operators and distributors, three-phase power is far more economical to generate than single-phase power.  Worldwide, all commercial electric power is created in generators configured as 3-ϕ machines.  The phases are designated as “Phase-1,” “Phase-2,” and “Phase 3;” this terminology can also be “Phase-A,” “Phase-B,” and “Phase C.”  Three-phase derived power is of special interest for boaters with 120V/240V, 50A shore power connections since it results in 120V/208V voltages.  More details in the section on “Special Situations.”

Single-phase AC is the type of electric service found in virtually all single family residential applications because it is easily derived from three-phase distribution systems, in two ways.  The first is to connect a load between any one of the phases of a three-phase service and a suitable electrical return point, usually the common of a 3-phase wye configuration.  This is how residential neighborhoods are serviced.  The second way to obtain single phase AC is to connect the load between any two phases of the 3-phase distribution system.  This is common in commercial applications and in apartment and condo buildings, but not in single family residential services.

US Utility Voltage Standards vs. Common Language

In the US (throughout North America), just what voltage standards do we have for residential and light commercial use?  Interesting question.  Is it 110V, 115V or 120V?  Is it 220V, 230V or 240V?  When people speak about residential voltages, it’s quite common to hear one or more of these numbers.  In actuality, they all mean the same thing.

Standardized utility voltages evolved over the years from 110V/220V systems to 115/230V to 117V/234V to 120V/240V.  In the 1970’s, the American National Standards Institute (ANSI) adopted the now current 120V/240V voltage standard via ANSI National Standard C84.1-1970.  This standard specifies two voltage ranges which included a specification for “service entrance voltage” and a standard for the voltage that would appear at user attached devices, called “utilization voltage.”   “Service entrance voltage” is measured at the meter, and “utilization voltage” is measured at the terminals of attached equipment.  The occurrence of service voltages outside the specified range (brownouts) was intended to be infrequent.

Following is the chart from ANSI C84.1.  The Range A service voltage range is plus or minus 5% of nominal. The Range B utilization voltage range is plus 6% to minus 13% of nominal.

Screen Shot 2020-06-06 at 14.11.39

The occurrence of service voltages outside the Range A limits should be infrequent. Household equipment is designed and rated to give fully satisfactory performance throughout this range.

Range B includes voltages above and below the Range A limits that necessarily result from practical design and operating conditions in utility or user systems, or both. Although such conditions are a part of practical operations, they should be limited in extent, infrequent, and of short duration (brownouts). If they occur on a repetitive or sustained basis, corrective measures should be undertaken within a reasonable time to improve voltages to meet Range A requirements.  Household equipment is designed to give acceptable performance in the extremes of this range of utilization voltages, although not necessarily as good performance as in Range A.

Table 1, below, is useful for an understanding of the relationship of power supplied by a power utility and the standards to which household appliances are manufactured.  The “Nominal” column is what we always talk about in common, ordinary discussion.  The “Service” and “Utilization” column are as discussed above.  The “Nameplate” column shows the voltage that will appear on an item you buy, such as refrigerator, dishwasher, washing machine, air conditioner, TV, Stereo, drill press, air compressor, etc.  The “NEMA” column (National Electrical Manufacturers Association) shows the tolerances used by manufacturing designers in creating the devices you buy.  These are all slightly different perspectives on the same thing.

Screen Shot 2020-06-06 at 14.38.46

There are two net messages here.:

  1. The voltage delivered to any residence will vary throughout the day, throughout the month, and throughout the year.
  2. The equipment in the residence is made to tolerate variation in utilization voltage.

This is very much more important to appreciate and understand when the scene shifts to boats in a marina, or more completely, when the scene shifts to boats cruising from place to place with widely different shore power services.

Residential Neighborhood

For simplicity, I started with power as delivered to single family suburban homes excluding light commercial buildings.  Light commercial buildings (condos, townhouses, apartments, offices, stores, and marinas) can all be served with single phase AC electric service, just as single family residences are, but more commonly, they are served by 3-phase utility service.  I will talk about these buildings later in the section on “Special Situations.”

Utility company power transformers have input sides, called the “primary,” and output sides, called the “secondary.”  Physically, both the primary and the secondary coils of the transformer are independent windings of wire wound around an internal metal core.  The windings are electrically isolated from each other; i.e., “insulated” from each other, but are “coupled” to each other by a shared magnetic field.  As the incoming primary voltage rises and falls, the magnetic field in the metal core strengthens and weakens.  As that magnetic field strengthens and weakens, voltage appears at the secondary.

Residential Single Phase :Street” Transformer (Typical)

The “load” for the transformer outside your house usually consists of four or so residential homes.  Throughout North and Central America the transformer is matched to the primary voltage to produce 120V/240V at the secondary.  The utility company transformer reduces the primary voltage to the residential requirement.  The range of transmission system primary voltages in a three-phase grounded wye configuration include; 34,500/19,900 volts; 22,900/13,200 volts; 13,200/7,620 volts; 12,470/7,200 volts; and, 4,160/2,400.  The first number in these number pairs represents the phase-to-phase voltage; the second number represents the phase to neutral voltage.  A single phase primary in a residential neighborhood is most commonly 7,200 volts, measured phase-to-neutral.  In rural residential primaries, 13,200 volts is common.

Transformer coils can be built with one or more “taps” on both the primary and secondary windings (coils).   The secondary winding of a residential power transformer is built with a single tap at the electrical midpoint of the coil.  This configuration is called a “center-tap.”  The three wires that come to a single-family residential  home from the utility pole are the two end-points of the secondary coil and the center-tap.  That center-tap conductor becomes the “neutral” within the building’s distribution wiring.

In the world of the electrical craftsman (electrician), it is desirable practice in a residential building or boat to have about ½ of the total household load attached to each side of the service transformer.  This practice balances the load on the secondary windings of the transformer on the street, and balances the concentration of heat that builds up within the windings and metal core of the transformer.  Transformers are oil cooled, and under heavy loads, they can get very hot.  Thus, balancing heat dissipation is crucially important in periods of very high electrical demand.  Days that are 104°F on the Chesapeake Bay or -30°F at International Falls are not times you’d want the transformer that serves your home to fail!

The definition of a North American residential standard power distribution system is a “single-phase, center-tapped, three-wire” service (alternatively, “single-phase, center-tapped, three-pole” service; in this case, the term “pole” represents a current carrying conductor.  Other common terms for this systems include “grounded neutral” and “split phase.”  The three parts of this definition are:

  1. single phase
  2. center-tap (gives rise to the system “neutral;” “grounded neutral”)
  3. three wires (two “hot” and one “neutral”)

From time-to-time, professional electricians and DIY lay technicians incorrectly refer to the residential “single phase, center tapped, three-wire” configuration as consisting of two phases.   The “evidence” is that one leg, “L1,” is 180° out-of-phase with the other leg, “L2.”  While “true,” this misleading factoid is a measurement curiosity caused by performing the electrical measurement from an inappropriate reference point.  Voltages from the two halves of our residential service will appear to be out-of-phase if measured with an oscilloscope from Neutral to “L1” and then from Neutral to “L2.”  The false appearance is the result of looking at the secondary of the transformer with reference to its center tap rather than across the entire winding.  This measurement curiosity is not present if the secondary is measured from “L1” to “L2” (or vice versa).  Think of it this way.  There is only one magnetic field alternately rising and falling in the transformer, driven by the rise and fall of the single-phase input at the primary.  That is the defining characteristic of “single-phase” equipment.  In a three phase device, there are three independent magnetic fields rising and falling within the equipment.  That is the defining characteristic of 3-phase equipment.  This distinction becomes extremely important when describing rotational torque in a 3-phase motor.

The above discussion is somewhat of a “technicality” issue, which has no practical importance in real life, and can safely be ignored!  When I was a pup, and first worked for an electrician in the early 1960s, I learned to refer to the two residential “hot” lines as “legs” instead of “phases.”  Doing so distinguishes the in-residence wiring from the conductors of the utility distribution system.  Frankly, except for the concepts involved, it’s really not important how you refer to this as long as you don’t let it confuse you!

Service Entrance – Single Family Residence

So now we understand that the electrical service entering a single family residence is a “single-phase, center tap, three-wire” service.  In our single family residence, if there are overhead wires and utility poles in the street, the three wires coming from the transformer are routed to a weather head or anchorage on the home, where they are spliced to wires leading to the electric meter.  In most jurisdictions in the US, the wires coming from the street are owned by the utility company.  The weather head, meter box and the wires from the “street splice” to the meter box are customer-owned.  The meter itself is owned by the utility company.

The customer-owned wire to the electric meter and from the meter to the main disconnect panel inside the building is comprised of two insulated wires (usually black) surrounded by a wrapping of bare wire strands.  This entire cable assembly is insulated as a single triplex unit.  This cable has a flat rectangular cross-section and is known as “Type SE,” or “Service Entrance” cable.  The two hot lines are routed to the input side of the “main” circuit breaker in the main disconnect panel.  The uninsulated neutral wire of the Service Entrance cable is routed to the neutral buss bar in the main panel.  The neutral buss bar is insulated from everything else in the service disconnect box, including the metal of the box enclosure, itself.  If the residence has an underground service, wires from a transformer located on a ground-level concrete pad will all be individually insulated wires rather than a triplex assembly.  They will be routed through underground conduit into the electric meter and then to the service disconnect panel.

The output side of the main circuit breaker in the disconnect panel is attached directly to metal “buss bars,” to which individual branch circuit breakers are fitted.  These buss bars are referred to as “L1” and “L2,” because they are on the overload-protected load side of the panel’s main circuit breaker.  The input side of the main disconnect breaker is referred to as the “Line” side and the output side is referred to as the “Load” side.

Electric Circuit “Wire” Naming and Identification

All electric circuits require two conductors (wires); one outbound from the source to the load, and one returning from the load to the source. The pair of conductors that lead current to and from the source of power are both called “Current Carrying Conductors.”

In single phase 120V residential circuits in North America, the two conductors are named for on their role in the circuit. The conductor that is considered to be the energized (power suppling) conductor is called the “Ungrounded Conductor,” or “Line 1,” or the “hot” conductor. By code and convention in North America, “L1” is black in color. The other conductor in a 120V circuit is considered to be the return conductor. It is called the “Grounded Conductor” (for reasons explained shortly), or the “Neutral Conductor,” or simple the “neutral,” and it is white in color.

In single phase 120V/208V and 120V/240V circuits in North America, there are two “Ungrounded Conductors.” They are commonly called “Line 1” and “Line 2.” “L1” is black, and “L2” is red. In these circuits, there is also a “Grounded Conductor,” always referred to as the “neutral,” and white in color.

In electrical engineering, “earth” is the single reference point in an electrical circuit from which voltages are measured and which provides a direct physical connection to the earth. Since the 1950s, the code for AC systems in buildings has required safety ground. This safety ground is a network of conductors that attach to every outlet, switch plate, ceiling fan, luminary fixture and wired appliance in the residence.  In a residential application, there are one or more copper rods driven into the ground outside the building.  The “System Grounding Conductor” (wire) is usually of bare #6 or #4 AWG stranded copper wire, and is routed from the buried ground rod(s) to a ground buss bar located in the service disconnect panel.  That buss bar is physically mounted on, and electrically connected to, the service disconnect panel’s metal box enclosure.  All of the ground wires that come from outlets and appliances everywhere in the building are routed to this buss bar.

In the NEC, each individual conductor that is a component of the “ground system” network has its own specific name, but for purposes of understanding concepts, the term used here will be the “ground conductor,” or “safety ground.”  The purpose of the safety ground conductors is to provide a low resistance fault-clearing path, so under normal conditions in a properly wired electrical system, the safety ground conductors DO NOT carry current. The ground conductor ONLY carries current when there is a “fault” in the system. In buildings on land, the ground conductor is often bare copper wire. On boats and in appliances, the ground wire is solid green in color, or green with a yellow tracer.

NEVER, NEVER use wires of the wrong color for the wrong purpose in a circuit. In new work, always buy the correct color of primary wire. Personnel safety and equipment safety depends on colors being correct! It is code-legal to “change” the color of a conductor in cases where that cannot be avoided. Changing the color of a wire is accomplished by wrapping electrical tape of the proper functional color for a distance of several inches at BOTH ENDS of the wire having its color changed. If ever that is found in existing work, DO NOT DISTURB that wrap of tape.

However tedious this discussion seems, an understanding of wiring terminology and color conventions is important to understanding electrical installation instructions for many different types of electrical equipment on boats, and to understanding the host electrical systems, themselves.

Main Service Disconnect Panel

We know from earlier discussion that the “neutral” in the building is a free-floating return line for power that arrives from the transformer hot lines.  But a free-floating return point is unlikely to be at “zero” volts, which is required to avoid electric shock in the home.  The NEC requires that the neutral line in a residence be bonded to earth ground “at the derived source of the electricity.”  For a home, the “derived source” is defined to be the main service disconnect panel.

In one design of main service disconnect panel, there is a buss bar dedicated to collecting branch circuit neutral conductors and a physically separate buss bar dedicated to collecting safety ground conductors.  In this style panel, there is a screw – usually dyed green in color – in the neutral buss bar. That screw is the “system bonding jumper,” or “bonding screw.”  This design allows the panel to be used either as a main disconnect panel or as a sub-panel.

If the disconnect panel is to be used as the Main Disconnect Panel, the bonding screw must be seated into the panel’s metal enclosure housing to electrically “bond” the “neutral” buss bar to the “safety ground” buss bar.  That screw is not for any mechanical purpose; it is the electrical bridge that make the “neutral” to “earth ground” connection.  THIS IS A CRITICALLY IMPORTANT SAFETY FEATURE.  NEVER OMIT OR REMOVE THE BONDING SCREW!

Sub-panel(s)

Sub-panels, a special case of residential switchgear, are used for several reasons:

  1. reduce the number of wire runs from the main service disconnect panel,
  2. manage the round trip length for long branch circuit wiring runs,
  3. manage the number of wires run in hidden chases/raceways/conduits, and
  4. reduce the cost of the installation.

The NEC does not limit the number of sub-panels that may be installed in a residential electrical system. Larger residential systems may have sub-panels located in several places around the home; ex: attached or detached garage, detached “guest quarters,” workshop, greenhouse or yard shed, pool house, Man Cave, She Shed, attic-space mechanical service (air conditioning compressor or attic vent fans), etc.  To install a sub-panel in residential applications, a single, appropriately sized 4-conductor cable, “Type SER,” is run from the main service entrance panel to the sub-panel (red arrow, below).  This 4-wire configuration carries “L1,” “L2,” “N” and “G” to the sub-panel switch box.  Because the sub-panel is subordinate to the main disconnect panel, the neutral-to-ground bonding screw is NEVER used in any sub-panel switch box.  By definition, the sub-panel is not the “source” for these branch circuits.  The main disconnect panel remains the “defined source” of the circuit.

The configuration of sub-panels in a residence is exactly analogous to the configuration of a boat attached to a marina shore power pedestal.  Notice the 240V, 3-pole, 4-wire feeder (red arrow) that connects the residential Main Disconnect Panel to the remote sub-panel.  This feeder is exactly analogous to the 240V/50A shore power cord of a boat.  The sub-panel “feeder cable” is “Type SER.”  It contains three current-carrying conductors and a safety ground.  Rather than the flat, rectangular cross-section of “Type SE,” “Type SER” cable features a round cross-section.  A boat’s “feeder cable” (shore power cord) is “Type SO” or “Type SOW,” which are very flexible cords.  Net: a boat looks like a sub-panel to the marina’s shore power system, and that is why the ABYC electrical standard seems so closely aligned with the requirements of the NEC.  Notice also in the drawing that the sub-panel safety ground leads back to the neutral buss in the main panel.  The neutral-to-ground bond is made only at the “derived source,” which is the Main Disconnect Panel.   Likewise, on a boat connected to shore power, there should never be a neutral-to-ground connection anywhere on the boat.  In both cases, the neutral-to-ground connection is made at the “derived source,” which is the main distribution panel in a residence, analogous to the marina shore power system for a boat.

Note: This main disconnect panel drawing shows a single buss bar which is shared by the neutrals and the grounds of branch circuits.  This arrangement is an NEC-compliant variation in a main disconnect panel.  Many main disconnect panels and all sub-panels will have physically separate busses for the neutrals and the grounds.

Branch Circuits

Branch circuits are where useful work gets done in the home.  There are three use cases:

  1. Between legs “L1” and “L2” alone, without “N,” we can power 240VAC, two-wire (two-pole) appliances; for example, the 240V motor of a deep-well pump, 240V baseboard electric heat radiator(s), or a 240V hot water heater.
  2. With “L1,” “L2” and “N,” we can power 240V, three-pole appliances; these appliances require 240V for some internal functions and 120V for other internal functions; for example, an electric dryer, range cooktop or oven; all of these appliances require 240V for the heating elements, but 120V for the motor and control circuits.  Or, central air conditioning system, which requires 240V for the compressor, but only 120V for the control circuits.
  3. Finally, with either “L1” or “L2” alone, and “N,” we can power the entire panoply of 120V, two-pole household loads; oil or gas furnace, dishwasher, incandescent and florescent lighting, computers, printers, routers, wireless telephones, TVs, VCRs, stereo, refrigerator, freezer, microwave oven, coffee maker, toaster, crock pot, waffle iron, blender, mixer, hair dryer, steam iron, battery chargers, shop tools, CPAP, oxygen concentrator, etc; you get the idea!

Branch circuits originate at a circuit breaker located in either the main service panel or a subordinate sub-panel.  Branch circuits feed either convenience outlets or feed into the attachment enclosure of a permanently installed appliance.  For convenience of installation and maintenance, the individual black, red, white and bare wires of a branch circuit are packaged together within a sheath of plastic outer insulation.  Most residential wire sold in big box, hardware stores and home centers is “Type NM,” meaning “non-metallic.” This is often called “Romex.”  “Type NM” intended to power 120V circuits is called “two-wire with ground,” or “two-pole, three-wire.”  “Type NM” intended to power 240V circuits is called “three-wire with ground,” or “three-pole, four wire.”    Another common residential wire is “Type AC.”  “Type AC” has an armored metallic sheath around the individual colored conductors instead of a plastic outer sheath.  “Type AC” is used for furnace controls for LPG and oil burners, hot water heaters and other appliance in an equipment room or basement, as well as when installed in areas exposed to being physically disturbed or damaged, such as workshops or garages.  Carefully match the wire you buy to the application you have, based on NEC and local electrical codes.

In the U. S., the color of the insulation on individual wires is important; “L1” is black, “L2” is red, “N” is white and “G” is uninsulated copper in convenience and appliance circuits, but can be green or green with a yellow tracer when insulated.

Occasionally, you may encounter a wire in a service disconnect panel or a junction box that has a piece of electrical tape of another color  conspicuously wound around it near its connecting end.  In a residential building, you may see red or black electrical tape wound on a white insulated wire, or you may see a piece or white electrical tape wound on red or black insulated wires.

Do not remove these pieces of tape; they are not an accidental left-over!  It means the installing electrician has “changed” the meaning of the base color of the insulation of the wire.  In residences, the most common place to find it is in wall boxes containing switches that control lighting or fans from multiple doorway locations, or wall boxes at the top and bottom of staircases.   If you ever see this, always triple-verify how the wire is actually being used before proceeding or disturbing the connection.

I have spent a lot of time talking about the current that arrives at the load in one of the energized conductors, “L1” and/or “L2,” and returns to the source in the neutral, “N.”  I have not discussed the use of the green ground wire, “G.”  In a correctly wired, normally operating home or boat AC electrical system, the ground wire should never have any current flowing in it.  The purpose of the safety ground wire is to provide an emergency path for current in order to trip the supplying circuit breaker to remove power from a faulting circuit.  By definition, current flowing in a safety ground is symptomatic of an electrical fault condition.  Fault currents originate from the hot line(s), but return to the source in the safety ground instead of the neutral.  This condition is also known as a “ground fault.”  Never use wire covered with green insulation as a current-carrying conductor.

Circuit Breakers

Contrary to popular belief, circuit breakers/fuses do not protect attached loads!  Circuit breakers do not protect TVs, entertainment systems, computers, microwaves, coffee pots, pumps or compressors.  CIRCUIT BREAKERS/FUSES PROTECT THE POWER-CARRYING WIRING THAT IS HIDDEN IN WALLS AND/OR ENCLOSED IN CHASES, RACEWAYS AND CONDUIT THROUGHOUT YOUR HOME OR BOAT!  They protect the WIRING of your home/boat.  This is a critically key concept.

When wires overheat, their colored insulation can melt, exposing the live conductor.  At that point, energized conductors can touch other now uninsulated conductors, and sparks can fly.  Wires in closed spaces, unusually warm spaces, or chases/raceways/conduits warm up more than wires in un-congested, cool, spaces where there is plenty of air circulation. Overheating softens the insulation.  Wires can get so hot that they will literally melt and can weld themselves together.  This process can cause adjacent nearby wood and composite building materials to burst into flame.  So, circuit breakers protect wires from overload, and therefore, protect the insulation from overheating, melting, failing and causing fires.

There are several common types of circuit breakers, and several manufacturers of circuit breakers and compatible service disconnect panels.  Circuit Breakers for 120V circuits are singe-wide; for 240VAC, they are “stacked” or “doublewide.”  Doublewide breakers have mechanically linked operating levers, and must be doublewide so that they can be physically installed in a service panel in a way that allows them to mate to both the “L1” and the “L2” buss bars at the same time.   If one leg of a 240V circuit – say, “L1” – develops a fault that causes the circuit breaker to trip, the mechanical link causes the other leg – in this example, “L2” – to also be disconnected from it’s source.  Never remove the mechanical linkage between doublewide breaker operating levers.

Switchgear on Boats – Residential vs. Marine-certified

Circuit Breakers should be selected based on the size of the wire they protect.  A 15A circuit breaker protects #14 AWG, Type NM cable; a 20A breaker protects #12 AWG Type NM, and a 30A breaker protects#10 AWG Type NM.  These numbers are based on the 60℃ temperature rating of “Type NM” wire.  Wire ampacities are higher with the 105℃ temperature rating of “Type BC5W2” boat cable.

Circuit breakers used for “over-current protection” (OCP) have rating of 15A, 20A, 30A or 50A.  That said, modern, sophisticated circuit breakers actually carry several ratings.  In a true short circuit, an over-current fault can instantaneously be as high as several hundreds of amps.  By arcing, that extreme amount of current can weld the contacts closed and permanently damage the circuit breaker’s contact points, rendering the breaker inoperable.  Circuit breakers and all switching devices carry an “Ampere Interrupt Capacity” (AIC) rating.  AIC is the amount of current the device can interrupt without being damaged by arcing.

Modern circuit breakers can also have multiple purposes.  Besides OCP, one added purpose is “Ground Fault Protection” (GFP) and another purpose is “Arc Fault Protection” (AFP).  GFP breakers contain a circuit that compares the amount of current being delivered in the hot wire(s) to the amount of current returning in the neutral.  Any difference in outgoing and returning current is a “ground fault.”  Household “Ground Fault Circuit Interrupter” (GFCI) breakers are designed to trip “off” if the difference between supplied and returned current is as little as 4mA – 6mA.  “Equipment Leakage Circuit Interrupters” (ELCI) onboard boats – and Equipment Protective Devices (EPD) on dockside pedestals – protect the whole boat, as a sub-panel.  ELCI/EPD are designed to trip “off” in less than 100 mS if the difference between supplied and returning current exceeds 30mA.

Finally, for use on gasoline powered boats and environments of potentially explosive gas, circuit breakers (and other electrical switching devices) must be rated as “ignition protected.”  This means that any internal arcing (sparking) caused by the contacts opening under load must not be able to come into contact with any airspace outside the breaker’s enclosure.  If explosive gasses were able to infiltrate the breaker’s enclosure, the vapors would be able to cause an explosion.  Of course, common residential circuit breakers are not made to the standard of “ignition protected” devices.

In general, in my opinion, it is bad practice to use “big box” and hardware store electrical switchgear equipment, circuit breakers or wire made for residential applications on a boat.  Residential switchgear is not made to withstand humid, salt-containing air, is not suited to the materials properties required by ABYC, and is not equivalent in temperature ratings for the ampacities of given conductor sizes.  NEVER, NEVER use solid core household wire on boats.

Aggregate Electrical Load – Residential Building

“How  much electrical “stuff” can we run “all at once” in our single family residential home?”  This is a key question for both residential applications and boats.  For boaters, it relates directly to discussions about 30A and 50A shore power cords and inlet wiring sizes.

Today, if you have a home of 2000 ft2 or more with an oil or gas-fired furnace, you’ll have a service entrance with at least a 200 amp service capacity.  If your home has electric baseboard heating and/or central air conditioning, it’ll probably have a 400 amp capacity. In the 1960s, we simply didn’t have as much “electrical stuff.”

What does it mean to “have a 400 amp capacity electrical service?”  In a moderate-sized residential building, if the individual capacities of all of the branch circuit breakers in your residential service disconnect panel were added up, there would probably be between 500 and 800 amps of distribution capacity.  For example:

8 – 30 amp double pole breakers for baseboard heating
1 – 50 amp double pole breaker for the range/oven
3 – 20 amp single pole breakers for the dishwasher, washer, and microwave
1 – 30 amp double pole breaker for the clothes dryer
1 – 40 amp double pole breaker for the hot water heater
1 – 40 amp single pole breaker for that great air compressor in the garage
20 or more – 15 or 20 amp single pole breakers for convenience outlets
1 – 50 amp double pole breaker for the air conditioning compressor

Hmmm…   Adds up to 810 amps (+/-) of branch circuit distribution capacity.  Take out the baseboard heating and you still have 570 amps.  However, that service panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at maximum breaker capacities.  Remember, breakers protect wires, so the individual breaker capacity is to protect the wire, not the attachment.  What happens if you exceed the capacity of the 200A/400A main breaker?  Well, in that case you’d blow the main breaker, but without blowing any of the individual branch circuit breakers.  Hmmm…

So to the question, “what does it mean to have a 200A or 400A electrical service?”  A “200 amp service” means that the installed utility-owned drop from the street, the conductors of the ”Type SE,” 3-wire service entrance cable to the electric meter housing, the conductors from the electric meter to the service disconnect panel, the service disconnect panel itself, and the earth ground connection are all sized and designed to operate in a safe manner when handling up to 200 amps for a 200A service, or up to 400A for a 400A service. If you exceed that capacity, that set of essentially unprotected electrical components may fail.  In effect, 200A/400A is the “ampacity” of the unfused and unprotected service entrance feed components.  So even though you have 500 to 800 amps of branch circuit load attachments, if you never exceed a combined aggregate load of 200 total amps, the distribution box will serve you just fine.  If every you do blow the main 200A/400A breaker in the home, have the cause determined by a qualified electrical professional!

Aggregate Electrical Load – Boat

The previous analysis of loading a residence main disconnect panel applies in exactly the same way to boats.  Most cruising-sized boats with 30A shore power will have well in excess of 30A of branch circuit capacity; likewise, boats with 50A shore power will have proportionally more branch circuit capacity.  That power is delivered onto the boat through a (30A)(50A) onboard main disconnect breaker, or compatible ELCI.  As with the residence case, the service distribution panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at their maximum breaker capacities.  If you exceed the maximum main breaker capacity, you blow either the main disconnect breaker, or the Shore Power pedestal breaker, generally without blowing any of the individual branch circuit breakers.

The ABYC requires an AC Main Disconnect Circuit Breaker within 10 feet of the shore power inlet.  Nothing is allowed to be connected ahead of that main disconnect breaker except the actual shore power inlet connector.  Recall, the purpose of circuit breakers is to protect wiring, and in particular, wiring hidden from view, and away from reasonably easy access, and running through spaces containing combustable materials.  The AC Main Disconnect Breaker protects the boat’s main inlet wiring (the boat’s “service entrance cable,” if you will) up to the main distribution panel that serves the boat’s individual branch circuits. Remember, the ampere rating of the disconnect breaker must be matched to the ampacity of the wiring between the power inlet plug and the main disconnect panel on the boat.  In the case of boats, the wiring installed by the boat manufacturer should reflect what the naval architect sped’ed for the boat.  Remember, the wires we’re talking about provide power to the AC circuit breaker panel of the boat, and carry the total aggregate current load for the whole boat.  Sizing shore power cords smaller than necessary could be dangerous.

AFCI and GFCI-protected Protection

Since 2008, the NEC has constantly extended the AFCI requirement to now include all habitable areas of a home, including kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar places.  Some states have modified these requirements when adopting the NEC as statewide regulatory code (building codes of all kinds are done on a County-by-County basis in Maryland).  Check local building codes before proceeding.

Since 1971, the NEC has continually expanded the coverage requirements for GFCI protection. Today, GFCI protection is required in all “wet” locations in residential buildings, which includes bathrooms, outdoors locations, rooftops, crawl spaces, unfinished basements, kitchen countertop areas, sinks, laundry areas, bathtub/shower stall areas, boathouses, locker rooms, pool areas: you get the idea.  On boats, the ABYC requires GFCI-protected outlets in heads, galley, machinery spaces and everywhere on the weatherdeck.

Should you wish to retrofit AFCI and GFCI-compliance into an older home (a good idea), a reasonable approach is to replace the conventional circuit breakers in the main  disconnect panel or sub-panel that serves affected branch circuits with combination AFCI/GFCI-protective circuit breakers.  That way, all outlets served by that circuit breaker are AFCI-protected and GFCI-protected.  Combination breakers are available from many manufacturers for about $35 – $45 apiece (as of January, 2018).  Discounts are available for volume purchases.   On the boat, physically compatible GFCI-breakers are not generally available, so GFCI-protected outlets are recommended.

GFCI-protected devices do present some unintended consequences.  A common scenario is for boaters to use adapters to enable a 30A or 50A shore power cord to use a standard 15A or 20A, 120V GFCI-protected utility outlet on a dock.  This provides power for a fridge, a battery charger, and maybe a reading lamp, for a night or two.

In the case of deteriorated, cracked insulation on a shore power cord lying in the water, a ground fault current could easily be large enough to trip a GFCI breaker, and that fault would not go away over time.  That condition is a true ground fault.  Not all trips are caused by true faults.  Sometimes, electronic components (capacitors and inductors) within the familiar portable computer “power bricks” can cause “momentary” surge currents that can trip sensitive GFCI protection devices.  Insulation breakdown on blower motors, pumps  and air conditioning compressors, as well as aging hot water heater elements, can cause transient power leaks.  Power spikes on power lines can trip GFCI devices.  All GFCI implementations are exposed to false faults resulting in “nuisance” trips.  When attaching to GFCI-protected outlets, it’s a good idea to set all AC breakers “off” first, then plug in, then turn branch circuits “on” one at a time.

For marinas and boatyards, starting in 2011, the NEC has required ground fault protection on new construction docks (except residential, single family docks until 2017).  These devices are called Equipment Protective Devices (EPD), and are also subject to “nuisance trips.”  To reduce the incidence of nuisance trips, the NEC has adopted two accommodations to lessen the occurrence of false trips on docks.  First, the size of the leakage current – 30mA – that would cause a marine pedestal EPD to trip “off” is greater than (less sensitive than) a 15A/20A GFCI convenience outlet.   Second, the length of time (duration of) the leakage current needs to be present – up to 100mS – has been made longer.  Since 2011, the rollout of these EPD sensors at marinas has been slow, but they are beginning to appear in greater numbers, and all boaters should expect to see EPD protection of marine outlets on docks with increasing frequency over the next few years.  The electrical knowledge and skills found among dock staff are unlikely to resolve problems for those who do experience nuisance trips at a marina.  Particularly on holidays, weekends and off-hours, high-school and college summer help are not likely to be able to assist transient boaters.

“Nuisance trips” may or may not mean you have wiring errors or equipment faults on your boat, but the fact is, many boats do have wiring errors and equipment faults that until recently have been silent and non-symptomatic.  Obviously, “troubleshooting” this scenario could be very complicated and time consuming.  If you have the skills to do it yourself, it’ll cost lots of time.  If you hire a marine electrician to do it for you, it’ll cost lots of money.  Either way, it won’t be easy or inexpensive.  It may well be that you just have older switchgear equipment, like a reverse polarity light with a filament that provides a “leakage path” from “neutral” to “safety ground.”  This is not an unsafe condition, but it will trip some EPD devices.  What is nasty about this is that “your boat is at fault,” and that’s precisely what you’ll get from the marina operator.

Special Situations – Life’s Little Complications

There are two types of three-phase wiring configurations: “wye” (or “star”) and “delta.” Three-phase distribution systems are used in commercial facilities and larger industrial facilities.  Within this category, I include condos, townhouses, strip mall offices, shopping centers, marinas and boatyards.  So, consider for example the case of three-phase distribution systems feeding end-user attachments in a condo or apartment.

In our “single family suburban residence” model, we learned the US standard voltages of a “single phase, center tapped, three wire” service entrance would be 240VAC/120VAC.  For many technical and economic reasons, light commercial and multi-family residential buildings are supplied from a three-phase, wye-connected service.  In a wye configuration, a 4-pole, 4-wire distribution system comprised of  “ϕ-1,” “ϕ-2,” “ϕ-3” and “N” is delivered into the building.  What is finally delivered, in turn, to the individual occupancy units is a 3-pole, 3-wire feeder analogous to the single phase street feed.  It is not, however, derived from the secondary of a single phase transformer.  Rather, it consists of any two of the three phases that came into the building, together with wye’s “N.”  As an example, suite 100 may receive “ϕ-1,” “ϕ-3” and “N,” and suite 102 may receive “ϕ-2,” “ϕ-3” and “N,” and so forth.

In the wye configuration, the voltages delivered to individual occupancy suites are not the standard 240VAC/120VAC.  Between “N” and any of the phases, the suite would see 120VAC. But between the two phases, the suite would see only 208VAC.  This service is written on paper as “208V/120V Y,” to indicate the phase-to-phase voltage, “208V,” the phase-to-neutral voltage, “120V,” and the fact that the configuration is a wye connection, “Y.”  This practice is common enough in the US that household appliances built for 208VAC/120VAC are commonly available in retail outlets for condo and townhouse dwellers.

Fortunately, 240VAC/120VAC appliances connected to 208V/120V Y services will usually work. Many are made to tolerate the lower line-to-line voltage.  The downside is, appliance efficiency may be reduced.  The power available across the “L1” and “L2” lines of phase-to-phase connection service will electrically be only 85% of the power available from the full design voltage.  As boaters, we need to be aware that many marinas are configured in this way.  If a boat has 240VAC appliances aboard – air conditioning, hot water heater, range/oven, washer/dryer, etc. – those appliances will receive “low voltage” if the marina is configured to provide “208V/120V Y.”

The most significant impact might be to 240VAC pump and compressor motors.  With a low voltage on the appliance, efficiency will be compromised, and motor overheating might occur.  Three phase “Y” distribution configurations are common in marina’s.  Boats with one or two, 2-pole, 30A shore power connections would not be affected.  Those connections are 120VAC.  Those with two 30A shore power cords connected to a “Y” adapter into a 50A outlet on a pedestal are also unaffected.  That’s because even though you are bringing the two different phase lines aboard, your boat does not have any 208V/240V appliances, so nothing aboard is affected.  Boats that connect to shore power with 3-pole, 50A shore power cord are potentially affected, as that 3-pole, 4-wire connector provides 240VAC with the expectation that it will be used on the boat.  Without 240V appliances, there is no affect.

The only thing you, as a boat owner/operator, can do to protect your appliances is to measure, with your onboard volt meter(s), the line voltages (2xxVAC/120VAC) provided by the shore power pedestal, each and every time you hook up.  In this way, you will know what the marina is delivering.  I recommend you become meticulous about this.  If you are not receiving 240VAC – if you are receiving only 208VAC – you will have to make decisions about what to do next.  Do not expect the dock hands that help you tie up to know what they have. Some may, but I would assume many would not.  Frankly, even the marina manager may not know.

DC Electricity On Boats

About This Article

This Article discusses DC Electricity concepts and terminology at an introductory level.  There are always discussions on boating bulletin boards relating to DC power systems on boats.  This article is intended to help those with little or no background or training in electrical systems to understand those discussions.  I have included the most important sub-topics related to 12V and 24V “low-voltage” DC power distribution systems encountered by typical cruising boat owners.

Electrical Safety

There is one, and only one, absolute when dealing with electricity.  VIRTUALLY ALL ELECTRICITY CAN BE DANGEROUS TO PROPERTY AND LIFE.  Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.   The large batteries and large banks of batteries found on boats can produce explosive gasses and store enough energy to easily start a large, fatal fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity!  WEAR HEARING PROTECTION when working in noisy environments, with running engines or other loud machinery.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right test equipment and tools for a job…
If you are not sure you know how to use the test equipment and tools you do have…
Well, then, LEAVE IT ALONE until you do!

USE INSULATED TOOLS when working around electricity, and especially around batteries.   Batteries contain enormous amounts of stored energy.  Accidental contact of a metal tool across the terminals of a battery is an emergency situation.  The tool can actually weld to the battery terminals and be both too hot to touch and impossible to remove without external mechanical force. Whenever working around a battery, pre-plan to have a two foot piece of 2”x4” readily available at hand.  If the worst should happen, use the wooden 2”x4” to knock the metal tool away from the battery terminals.  DO NOT TOUCH the tool; assume it will be far too hot to handle with bare hands!  Once this cascade of events has started, the only way to stop it is to break the tool free of the battery terminals.  Otherwise, the battery will get so hot it will melt and may start a fire.

Be very wary of unfamiliar, pungent odors.  Transformers, motors and most electrical and electronic devices that are in the process of failing often heat up and cause insulating or potting materials to give off strong, pungent odors. TURN OFF POWER and use your nose to track down the source.  Treat this as a true emergency.  If you can find the offending device before it bursts into flame, you’re way ahead of the game!  Turning off the power will usually allow the device to cool off.  Do not restart the device!  Excessive over-heating often causes secondary internal damage that you cannot see.

What Is DC Electricity?

DC voltages at their source are characterized by 1) a stable voltage amplitude of 2) unchanging polarity; i.e., the polarity of the voltage between the supply and return terminals never changes.  One battery terminal is considered “positive” and marked with a “+” sign, and one battery terminal is considered “negative” and marked with a “-” sign.  Terminals are either “positive” or “negative” with respect to each other, nit the external world.  The “positive” terminal is positive with respect to the “negative” terminal; the “negative” terminal is negative with respect to the “positive” terminal.  This distinction is important in using a voltmeter to measure voltages.  A DC voltmeter will provide both the amplitude of the voltage that’s present and the polarity of the conductors or components between which the meter is attached.   The amplitude of the voltage can vary somewhat over time, as over the period of time that a battery discharges, but the polarity of that voltage between battery terminals does not change.  This is the fundamental difference between AC and DC electricity, and that difference leads to all of the technical advantages and disadvantages the different electricity technologies offer to users.

Key Electrical Concepts and Terms

The following are some terms regularly used in listserv posts and widely encountered in discussions of electrical systems and circuits.  Boaters will do themselves a great favor by learning these terms and understanding the concepts these terms represent.

  1. Source – Point-of-origin of an electric current.  Typically for DC systems, a battery or bank of batteries.  Electrical sources are “balanced systems,” in that whatever current leaves must return to the source on a one-for-one basis.  If a return path is not available, current cannot flow and useful work cannot be performed.
  2. Load – The components within an electrical system that consume electrical energy to operate; ex: lights, heating elements, motors and electronics.
  3. Circuit – a network of conductors and components carrying electric current from the source to the load, distributing current throughout the load, and returning current to the source from the load.  Circuits are always closed loops that originate AND terminate at the power source.
  4. Supply (or “B+”) – the current-carrying conductor that transports electric current from the source to the load where power is consumed.  In “negative ground” DC systems as required on boats, often called “B+.”
  5. Return (or “B-”) – the current-carrying conductor that returns power from the load back to the source.  In negative-ground DC systems as required by ABYC on boats, often called “B-.”  Analogous the the “neutral” conductor in AC circuits.
  6. Voltage/Volt – the unit of quantification of “Electromotive Force” (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive force is measure across two points in a circuit.  (Volt, millivolts)
  7. Ampere/Amp – the unit of quantification of current flowing through a particular point in an electric circuit.  (Amps, milliamps)
  8. Current – the flow of electrons through a conductor, or the flow of ions through a liquid medium such as salt water; electric current is what performs “work,” i.e., fulfills the purpose of a circuit.  (Ampere; Amp)
  9. Resistance – physical property of all electrically conductive media that acts to retard or impede the flow of electrons through it.  All conductors have resistance. (Ohm)
  10. Conductor (lead, line, cable) – circuit device that transports electric currents.
  11. Ohm’s Law – Mathematical formula that describes the relationships between voltage (V), current (I), resistance (R) and power (P) in a circuit.
  12. Power – the quantification of the amount of “work” that electric current performs in its application.  In purely resistive applications, this will be light or heat.  In turning a motor, this will be the amount of electrical energy consumed in creating torque.  (Watt) (appropriate torque unit)
  13. Ground – a) a universal standard earth reference voltage of “0” volts; b) conversationally, the portion of an electrical circuit to which all other parts are referenced.
  14. Common – any interconnected portions of circuit to which many other parts of an electrical system are also connected.  If reference is specifically to “ground,” this term references a “return” return path shared by many separate portions of an electrical system.  Example: the  positive and return conductors to flybridge nav instruments may be supplied by a “common” B+ power feed conductor (red) and wired with a “common” B- return (yellow) conductor.  Analogies: “ground,” “B+,” “B-” “buss.”  Opposite: “home run.”
  15. Neutral – a non-ground, normally current-carrying return path for electric currents; customarily used in the context of AC circuits.  In DC applications, B- conductors are analogous to the AC neutral.
  16. Fault current – a current flow that follows an abnormal and unexpected path from its source to its return point.
  17. Short circuit – an electrical fault condition resulting from the unintentional connection of a source directly to a return circuit or earth ground.  This unintentional connection often results in the flow of extremely large fault currents. The electrical system should be designed in such a way that fault currents are automatically interrupted by circuit breakers of fuses.  This condition may not cause overload protection devices (circuit breaker) to disconnect the source of power in “ungrounded” systems..
  18. Chase – enclosed spaces in a building or a boat through which wires are run to achieve access to remote locations
  19. Raceway, Conduit, Spiral Wire Wrap, Split Wrap – varieties of supplemental physical enclosure intended to protect electrical conductors from accidental physical damage, excessive ambient temperatures and vibration.
  20. Switchgear – a generic term for all equipment housings in which fuses or circuit breakers and similar disconnecting or switching devices are mounted.  This term is used across the electrical power industry, from generating stations to transformer yards to neighborhood distribution yards to commercial and residential locations.
  21. ABYCAmerican Boat and Yacht Council, Annapolis, MD.  An organization that produces a comprehensive set of safety standards applicable to boats and boat manufacturers, the marine insurance industry, surveyors, attorneys involved in litigation and boat owners.
  22. NECNational Electric Code; United States electrical design standards for Power Generating and Distribution Systems, state, county and community code regulations  and the electrical construction industry.
  23. NFPANational Fire Protection Association; organization that creates and maintains the NEC.

DC Circuits

Fundamental Concept

The essential components of all electrical circuits are:

  1. a source of electrical energy,
  2. a conductor that transports electric current from the energy source to a load,
  3. an electrical load, where useful “work” results when an electric current flows, and
  4. a conductor that transports the electric current back from the load to the energy source.

By definition, an electrical “circuit” must contain all four of the above elements.  All electrical circuits (DC or AC) originate as a pair of electrical terminals that are connected to power-consuming load devices by conductors (wires) of one type or another.  Electric current flows through a circuit.  If a complete electrically-conductive loop is not available from “source” through “return,” an electrical current cannot flow.  Switches, fuses and/or circuit breakers are used to create an incomplete electrical path from the source to the load.

An electric current is the aggregate of millions of migrating electrons (and ions in liquid media).  In DC circuits, it is “convention” to think of the electric current flowing from positive to negative.  This “convention” Is a “working agreement” across all electrical standards bodies, trades and professions.  By mutual agreement, all electrical diagrams of DC circuits and electronics circuits are shown with symbols that assume current flows from positive to negative.  It is a fact of atomic physics that electrons carry a negative electrical charge, so migrate from a more negative place to a more positive place.  As in most “conventional agreements,” as long as the convention is agreed and understood, the pesky facts of atomic physics can be overlooked and left to scientists.

Circuit “Common” Reference(s)

The term “common” applies broadly to circuit elements that are shared among all of the broader network of electrical attachments in a installed electrical system.  The supply buss (“hot, B+”) and the negative return buss (“B-”) are examples of common circuit elements.

Virtually all DC systems encountered by the general public are low-voltage circuits, generally 12-volts, occasionally 24V or 32V.  Examples are 12-volt motor cycle, automobile, light truck, lawn tractor, residential emergency generator, snow thrower or all-terrain vehicle starting batteries, and similar yard and garden devices.  Other low-voltage battery-operated devices include fire/burglar alarms, Uninterrupted Power Supplies (UPS) for computers and data networks, hand-held spot lights, wireless telephone systems and a very wide variety of portable tools.

For applications in the automobile, truck, and outdoor equipment sectors, the return terminal of the battery is typically attached to the metal frame of the vehicle/equipment upon which the  battery is mounted.  The frame is the “common” return path for all sub-circuits.  Electrical components (starter motors, blowers, horns, light sockets, solenoids, sensors, gauges, electronics, etc) have internal electrical return connections that attach to the vehicle’s frame.  The electrical connection is created when the component is bolted to the chassis of the vehicle.  No discrete return conductor is needed because the metal vehicle chassis is the common electrical return path.  This approach simplifies wiring and mechanical design, reduces component design complexity, reduces material and labor cost, and eliminates wiring and connector materials and weight.  The metal frame of a vehicle is perhaps the most obvious place where the term “common” would describe a broadly-shared circuit component.

There are several factors that affect the preceding discussion as it applies to boats:

  1. most small and mid-sized pleasure craft are wired with 12-volt DC systems; 24-volt and 32-volt DC systems are sometimes used;
  2. some medium and large-sized boats have hybrid DC systems of mixed 12V, 24V and 32V systems;
  3. fiberglass (fiberglass reinforced plastic, FRP) does not conduct electricity, so fiberglass boat construction does not provide a functional “chassis,” or “vehicle frame,” return path; and,
  4. electric currents of even the smallest magnitude flowing in metal hulls, metal stringers and/or metal frame members lead to corrosion of the metals, and are always undesirable on boats.

Electrical appliances and utility attachments intended for marine DC applications are designed to have at least two wires; one for the supply of current that originates in the source (B+), and one for the explicit return of that current back into the source (B-).

Ground

In all of the preceding discussion, I’ve intentionally referred to the “electrical return path” using that specific term.  In ordinary conversation, the term “ground” is often used to describe the return path of a DC circuit.  This is a technical “liberty” of conversation, since DC return paths are often not actually connected to earth ground.  “Ground” in this context is a term of convenience and convention.  The return path from a low-voltage DC load to its source (B-) is not inherently at zero volts with respect to its surroundings.  A battery held in hand or sitting in a dock cart has two terminals, but neither is referenced to it’s surrounding environment.

Consider a bird sitting on a high-voltage overhead wire in a residential neighborhood.  The wire is at thousands of volts with respect to the earth, and so is the bird’s body and all of it’s little body parts.  But, the bird is safe, the tiny electric currents that make the bird’s heart beat still work, because there is no return path from the bird’s body to enable a disruptive external current to flow.  As soon as the bird flies off, the voltage is gone.  The bird’s body voltage changes, but the bird’s heart still beats normally, and the bird survives, completely unaffected for the contact with that man-made high voltage.

Consider a car, then, that is mounted on rubber tires.  Since rubber is a fairly good insulator, it would be possible for a DC voltage to exist between the earth and the frame of the car.  Normally small, this voltage can be thousands of volts.  Readers who have ever visited or lived in cold climates are undoubtedly familiar with the static shock that can happen when exiting a the car.  That static shock was a blast of high voltage DC caused by the transfer of accumulated charge from the vehicle, through the body, to the earth.  (Well, for purists, electrons flow from the earth, through the body, to the vehicle, but to the shockee, that detail is uninteresting.)  Static electricity and lightening are the same phenomenon, only on a much different scale!  The possibility of static shock is why every gasoline dispensary in the country instructs drivers to remove portable gas cans from cars and place them on the ground before filling them.  Grounding the container disburses any static charge.

It is technically non-trivial to create a reliable earth ground on a car.  Some readers may have seen ground straps dangling from trucks and some cars.  Used mostly on trucks, those straps are intended to protect toll takers and others who might come into contact with the vehicle from static shocks and to provide a safe path to ground static charges.  It is obviously difficult to create a reliable earth ground on a boat, impossible on an airplane.  Historically, earth grounding was not regarded as an important design goal for DC electric circuits.  And of course, experience with low voltage DC equipment generally bears out that assumption.  We get shocks from the build-up of static, but we don’t get shocks when we step off the garden tractor or use the snow blower.  Who among us has never disconnected a car battery when standing on the ground, and that was not a shocking experience.

What this all implies is that, even though the DC return circuit may not actually always be at the electrical potential of earth ground, the DC return circuit in all of our familiar yard equipment, cars and SUVs is referred to in ordinary discussion as the circuit’s  “ground”.  This use of the term “ground” refers to the functional return path ground, not a safety ground.

Safety Ground

As society gained experience with electricity in the early and mid-20th century, it became obvious that there had to be a way to ensure the return path is always at “earth ground” potential in order to  avoid the possibility of personal harm or property damage resulting from accidental contact with electric power.  A safety ground is not required for a circuit to operate correctly, but it does provide other compelling benefits.

Consider a fiberglass boat.  Aboard, there are many parallel DC sub-circuits.  Water pumps, space and nav lighting, nav and entertainment electronics, windlass, thruster, the propulsion engine, etc.  They are all at distances from one another, and the fiberglass frame of the boat is non-conductive.  A safety ground in a DC system (if present) interconnects the external frames and metal cases of equipment, appliances and utility attachments (light switches, outlets, motors, electrical equipment, radios, etc.) to a known point of common potential.  That common point is always the negative terminal of the battery, and under some specific conditions, the water in which the boat floats or the earth itself.  A safety ground is separate from the functional return circuit, and always involves the installation of it’s own individual electrical conductors.

In service, a “safety ground” is never intended to carry current in normal operation.  However, in a circuit containing an electrical fault condition, a safety ground is intended to prevent a personal shock hazard or mitigate property damage risk by ensuring the electrical potential is at earth ground potential.  It is the “safety ground” that provides an emergency path that allows a circuit breaker to function and disconnect power.

Consider, for example, a bow thruster or an anchor windlass.  We would expect to have a battery positive connection to the positive (B+) terminal of the device’s motor solenoid, and a battery negative connection to the negative (B-) terminal of the device’s motor solenoid.  The motor would then be expected to operate correctly with just these two battery connections.   If we also had a separate conductor from the mounting frame of the device to the vessel’s bonding system, that would be considered the “safety ground.”  The thruster would run just fine without the safety ground, but the device could malfunction and place the frame at some non-zero electrical potential.

Vessel Design – “Grounded” vs “Ungrounded”

Designers of DC electrical distribution systems refer to them as either “grounded” or “ungrounded” systems.  The terms “grounded” and “ungrounded” refer to the presence or absence of a safety ground, not the functional return circuit.  A return path of electrons to their source is always required, but that return path is not always referenced to anything else!

There is valid debate among experts as to whether 12-volt, 24-volt and 32-volt boat DC systems can be of the “ungrounded” design or should be of the “grounded” design.  Today, DC grounded systems are not common.  However, new and emerging vessel propulsion systems containing large-horsepower (hP) diesel-driven DC generators and large-horsepower DC motors (systems analogous to diesel-electric train locomotives) are definitely high voltage applications (often between 600VDC and 1000VDC).  Faced with the emerging presence of true medium and high-voltage DC equipment on pleasure craft, this safety ground design choice is now specifically being re-evaluated in the American Boat and Yacht Council’s (ABYC) Electrical Technical Committee.  We await that outcome.

It is to the advantage of boat buyers and all boat owners to understand the low-voltage DC electrical distribution system.  It’s also an obligation of the buyer/owner to understand whether or not a medium or high voltage DC system is also present.  In the majority of fiberglass-hulled boats, it would be unusual to have a separate DC safety grounding circuit installed.  On some boats, nevertheless, one could encounter one of several possibilities.  The electrical system installation on any individual boat depends on:

  1. the prevailing electrical construction standards at the time of OEM fabrication, often related to prevailing standards of the international geography where the boat was built,
  2. how many people may have added to, or otherwise modified, the system over time, and
  3. the electrical skills those individuals who have performed electrical work in the highly specialized marine environment.

The possibilities aboard a vessel include:

  1. no low-voltage DC safety ground at all (most typical today),
  2. partial DC safety grounding on some parts of the system (not recommended; considered technically inadequate), and
  3. full DC safety grounding, vessel-wide.

The ABYC does not require that low-voltage DC distribution systems have a safety ground, but it does make “recommendations” as to how “grounded” and “ungrounded” systems must be interconnected with the vessel’s bonding system.

Polarity  – “Negative Ground” vs “Positive Ground”

Earlier, I pointed out that a battery held in hand or sitting in a dock cart has two terminals, but neither is necessarily referenced to ground.  All that can be said is there is a fixed voltage between the two battery terminals.  Whichever battery terminal is connected to the vehicle frame determines the polarity reference for that DC system.  If the negative battery terminal is connected to the vehicle chassis, the system is considered to be a “negative ground” system.  If the positive terminal is connected to the vehicle frame, the system is considered to be a “positive ground” system.  With the emergence of solid state electronics and economic pressure to reduce manufacturing cost by sharing components across brands, models and manufacturers, the modern automobile industry world-wide (at least since the 1980s) has standardized around negative ground systems.

The ABYC-approved, and by far the most common, DC systems found on pleasure craft in North America are “negative-ground” systems.  On a boat with other than negative-ground DC distribution system, the panels throughout the boat should be clearly marked to identify the manner of connection.  If there is any doubt, always use a voltmeter to confirm the configuration before disconnecting or otherwise making modifications to the system.